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Frontispiece 


TELESCOPIC  VIEWS  OF  THE  PLANETS. 


ELEMENTS    OF    ASTRONOMY 


BY 


SIMON   NEWCOMB,  PH.D.,  LL.D. 

FORMERLY  PROFESSOR  OF  MATHEMATICS  AND  ASTRONOMY 
JOHNS  HOPKINS   UNIVERSITY 


NEW  YORK-:- CINCINNATI-:.  CHICAGO 

AMERICAN    BOOK    COMPANY 


COPYRIGHT,  1900,  BY 
SIMON  NEWCOMB. 


«  -.'    BL.  oi^ASTitc*"  «  *.  / 

EDtiC/Cfl' ON"' D'EPT ;  ' 


Q643 


PREFACE 

Two  objects  have  been  kept  in  view  in  preparing  this  little 
book.  One  was  to  condense  those  facts  and  laws  of  the 
science  which  are  of  most  interest  and  importance  to  the 
general  intelligent  public  within  so  small  a  compass  as  not 
to  make  a  very  serious  addition  to  the  curriculum  of  the  high 
school  or  college.  The  other  was  so  to  present  the  subject 
that  as  little  formal  mathematics  as  possible  should  be  neces- 
sary to  its  mastery. 

Of  the  first  object  little  need  be  said.  The  typical  person 
constantly  kept  in  mind  has  been  the  inquiring  la}rman  seek- 
ing to  know  something  of  the  heavenly  bodies  and  their  rela- 
tion to  the  earth,  including  such  subjects  of  human  interest  as 
the  changing  seasons,  the  measure  of  time,  and  the  varying 
aspects  of  the  planets. 

The  second  object  involves  more  serious  questions.  Can  an 
idea  of  the  laws  and  phenomena  of  the  celestial  motions  be 
conveyed  to  a  pupil  who  has  not  completed  the  regular  course 
in  geometry  and  physics  ?  The  author  believes  that  it  can.  It 
cannot,  indeed,  be  denied  that  the  professional  astronomer, 
engineer,  surveyor,  and  navigator  who  are  to  make  astronomi- 
cal observations  and  computations  must  have  a  fairly  complete 
training  in  at  least  the  elementary  branches  of  mathematics. 
But  this  training  is  not  essential  to  him  who  desires  only  a 
command  of  general  ideas,  without  proposing  to  make  technical 
applications  of  the  science.  What  is  really  essential  are  those 
conceptions  of  motion  and  form  which  one  may  derive  from 
everyday  observation,  and  the  understanding  of  a  few  elemen- 
tary definitions  in  geometry  and  physics.  Our  modern  system 

M69857 


6  PREFACE 

of  education  wisely  endeavors  to  implant  such  conceptions  and 
to  teach  the  corresponding  definitions  at  an  earlier  age  than 
that  when  the  growing  youth  is  expected  to  commence  a  course 
of  formal  mathematics. 

The  author  hopes  that  the  early  chapters  are  the  only  ones 
that  will  offer  any  difficulty  to  an  intelligent  pupil  prepared 
for  a  high  school  course.  Here  it  is  believed  that  every  diffi- 
culty may  be  overcome  by  two  very  simple  measures  on  the 
part  of  the  teacher.  One  is  to  point  out,  approximately,  the 
actual  position  of  the  celestial  poles  and  equator  and  the  appar- 
ent diurnal  courses  of  the  sun  and  stars,  as  they  might  be  seen 
in  the  mind's  eye  from  the  schoolroom  or  the  field.  The  object 
of  this  is  that  the  learner  may  conceive  the  phenomena  he  is 
studying  as  if  seen  in  the  sky.  The  other  is  to  see  that  the 
learner  correctly  apprehends  the  meaning  of  the  figures  repre- 
senting points,  circles,  and  motions  on  the  celestial  sphere ; 
especially,  that  he  always  imagines  himself  looking  at  the 
objects  represented  as  if  he  were  at  the  center  of  the  sphere. 

For  this  last  suggestion  and  for  other  valuable  hints,  the 
author  takes  much  pleasure  in  acknowledging  his  indebtedness 
to  Mr.  Edward  P.  Jackson,  teacher  of  Physics  in  the  Boston 
Latin  School. 


CONTENTS 


CHAPTER  PA8F 

I.     RELATION  OP  THE  EARTH  TO  THE  HEAVENS  ....        9 

1.  Introduction.  2.  Ideas  of  Motion.  8.  The  Earth. 
4.  The  Celestial  Sphere.  5.  Perspective  of  Plane  and  Line. 
6.  Angular  Measure  on  the  Celestial  Sphere.  7.  The  Rela- 
tion of  the  Horizon  to  the  Celestial  Sphere.  8.  The  Diurnal 
Motion.  9.  Celestial  Equator  and  Poles.  10.  The  Meridian. 
11.  Diurnal  Motion  in  Different  Latitudes.  12.  Right  Ascen- 
sion and  Declination.  13.  Correspondence  of  the  Terrestrial 
and  Celestial  Spheres. 

II.     THE  REVOLUTION  OF  THE  EARTH  ROUND  THE  SUN         .         .       82 

1.  The  Earth  as  a  Planet.  2.  Annual  Motion  of  the  Earth 
round  the  Sun.  3.  How  the  Sun  shines  on  the  Earth  at  Dif- 
ferent Seasons.  4.  Apparent  Motion  of  the  Sun  —  The 
Zodiac.  5.  Seasons  in  the  Two  Hemispheres.  6.  The  Solar 
and  Sidereal  Years.  7.  Precession  of  the  Equinoxes. 

ILL    OF  TIME 48 

1.  Diurnal  Motion  of  the  Sun  and  Stars.  2.  Mean  and 
Apparent  Time ;  Inequality  of  Apparent  Time.  3.  Local 
Time  and  Longitude.  4.  Standard  Time. 

IV.     OBSERVATION  AND  MEASUREMENT  OF  THE  HEAVENS        .         .       56 

1.  Refraction  of  Light.  2.  Lenses  and  Object  Glasses. 
3.  The  Refracting  Telescope.  4.  The  Equatorial  Telescope. 
6.  The  Reflecting  Telescope.  6.  Great  Telescopes.  7.  Me- 
ridian Instruments.  8.  The  Spectroscope  and  its  Use. 
9.  Semidiameter  and  Parallax.  10.  The  Aberration  of  Light. 

V.     GRAVITATION 80 

1.  Force.  2.  The  Laws  of  Motion.  3.  Universal  Gravita- 
tion. 4.  Weight  and  Mass.  5.  How  the  Attraction  of  the 
Sun  keeps  the  Planets  in  their  Orbits.  6.  Centrifugal  Force. 

VI.  THE  EARTH 90 

1.  Figure  and  Magnitude  of  the  Earth.  2.  Latitude  and 
Longitude.  3.  Length  of  a  Degree.  4.  How  the  Earth  is 
measured.  5.  How  Latitude  and  Longitude  are  determined. 
6.  Density  of  the  Earth,  Gravity,  etc.  7.  Condition  of  the 
Earth's  Interior.  8.  The  Atmosphere.  9.  The  Zodiacal 
Light. 

VII.  THE  SUN 103 

1.    Particulars    about   the    Sun.      2.    Heat  of   the  Sun. 
3.  Spots  and  Rotation  of  the  Sun.      4.  Corona  and  Promi- 
nences.    5.  Source  and  Period  of  the  Sun's  Heat. 
1 


8  CONTENTS 

CHAPTER  PAGE 

VIII.     THE  MOON  AND  ECLIPSES .112 

1.  Distance,  Size,  and  Aspect  of  the  Moon.  2.  The  Moon's 
Revolution.  3.  The  Moon's  Phases  and  Rotation.  4.  The 
Tides.  5.  Eclipses  of  the  Moon.  6.  The  Moon's  Orbit  and 
Nodes.  7.  Eclipses  of  the  Sun.  8.  Recurrence  of  Eclipses. 

IX.     THE  CALENDAR 133 

1.  Units  of  Time.  2.  The  Julian  Calendar.  3.  The  Gre- 
gorian Calendar.  4.  The  Year.  5.  Features  of  the  Church 
Calendar.  6.  The  Hours. 

X.     GENERAL  PLAN  OF  THE  SOLAR  SYSTEM        ....     140 

1.  Orbits  of  the  Planets.  2.  Kepler's  Laws.  3.  Structure 
of  the  Solar  System.  4.  Distances  of  the  Planets ;  Bode's 
Law.  6.  Aspects  of  the  Planets.  6.  Apparent  Motions  of 
the  Planets.  7.  Perturbations  of  the  Planets. 

XL     THE  INNER  GROUP  OF  PLANETS 151 

1.  The  Planet  Mercury.  2.  The  Planet  Venus ;  Aspects 
of  Venus.  3.  The  Planet  Mars ;  Aspects  of  Mars.  4.  The 
Minor  Planets  or  Asteroids. 

XII.     THE  FOUR  OUTER  PLANETS 162 

1.  The  Planet  Jupiter.  2.  The  Satellites  of  Jupiter. 
3.  The  Planet  Saturn.  4.  The  Rings  of  Saturn.  5.  The 
Satellites  of  Saturn.  6.  Uranus  and  its  Satellites.  7.  Nep- 
tune and  its  Satellite. 

XIII.  COMETS  AND  METEORS 176 

1.  Appearance  of  a  Comet.  2.  Comets  belong  to  the  Solar 
System.  3.  Orbits  of  Comets.  4.  Remarkable  Comets. 
6.  Constitution  of  Comets.  6.  Meteors.  7.  Meteoric 
Showers. 

XIV.  THE  CONSTELLATIONS 191 

1.  About  the  Stars  in  General.  2.  How  the  Constellations 
and  Stars  are  named.  3.  Description  of  the  Principal  Con- 
stellations. 4.  Constellations  Visible  in  the  Evenings  of 
February  and  March.  5.  The  Early  Summer  Constella- 
tions. 6.  The  August  Constellations.  7.  The  November 
Constellations. 

XV.  THE  STARS  AND  NEBULJE 205 

1 .  The  Stars  are  Suns.  2.  Proper  Motions  of  the  Stars. 
3.  Motion  of  the  Sun.  4.  Motions  in  the  Line  of  Sight. 
5.  Distances  of  the  Stars.  6.  Variable  Stars.  7.  Double 
Stars.  8.  Clusters  and  Nebulae  ;  Clusters  of  Stars. 

XVI.     A  BRIEF  HISTORY  OF  ASTRONOMY        .  .     225 


INDEX 


ASTRONOMY 


»<**< 


CHAPTER. I 
RELATION  OF  THE   EARTH  TO  THE  ' 

L  Introduction.  —  When  we  look  at  the  sky  by  day  we  see 
the  sun;  by  night  .we  see  the  moon  and  stars.  These,  and  all 
other  objects  which  we  see  in  the  heavens,  are  called  heavenly 
bodies.  Astronomy  is  the  science  which  treats  of  these  bodies. 

The  heavenly  bodies  are  all  of  immense  size,  most  of  them 
larger  than  the  earth.  They  look  small  because  they  are  so 
far  away.  If  we  could  fly  from  the  earth  as  far  as  we  please, 
it  would  look  smaller  and  smaller  as  we  went  farther,  until  at 
a  distance  of  many  millions  of  miles  it  would  appear  as  a  little 
star.  If  we  kept  on  yet  farther,  it  would  at  last  disappear  from 
our  sight  altogether. 

If  we  lived  on  one  of  the  heavenly  bodies,  it  would  be  to 
us  as  the  earth,  and  the  earth  would  be  seen  as  a  heavenly 
body. 

In  trying  to  think  of  the  relation  of  the  earth  to  the 
heavens,  we  may  liken  ourselves  to  microscopic  insects  liv- 
ing on  an  apple.  To  them  the  apple  is  a  world,  than  which 
nothing  bigger  can  be  conceived.  As  this  continent  is  to  their 
apple,  so  is  the  universe  of  stars  to  our  world.  We  may  fancy 
how  their  ideas  would  have  to  be  enlarged  to  make  them  com- 
prehend the  relations  of  the  Atlantic  and  Pacific  oceans ;  and 
then  we  may  try  to  enlarge  ours  in  the  same  way  to  under- 
stand the  relations  of  the  heavenly  bodies. 

9 


10  ASTRONOMY 

2.  Ideas  of  Motion.  —  If  we  think  carefully,  we  shall  see 
that  we  can  never  know  that  any  object  is  in  motion  except  by 
comparing  its  position  with  that  of  some  other  object  supposed 
to  be  at  rest.     Inside  the  cabin  of  a  ship  on  a  smooth  sea  we 
are  not  able  to  decide  whether  we  are  at  rest  or  in  motion 
unless  we  can  look  out  on  the  ocean  which  we  suppose  to  be 
at  rest.     Even  then  water,  ship,  and  everything  on  the  ship, 
might  be  carried  along  by  the  Gulf  Stream  without  our  know- 
ing it.     This  -general  :fact  is  expressed  by  saying  that  all 
motion,  so  far  as  "we  can  define  or  know  it,  is  relative  ;  that 
is,  it  H  -referred  to  some  object  supposed  to  be  at  rest. 

It  follows  from  this  that  the  motion  of  an  object  may  be 
very  different  according  to  the  body  to  which  it  is  referred. 
Suppose,  for  example,  that  a  man  walks  from  the  front  to  the 
rear  of  a  railway  car  running  eastward  50  miles  an  hour. 
A  fellow  passenger  would  say  that  the  man  was  walking 
westward  at  the  rate  of  three  miles  an  hour,  because  his 
motion  would  be  referred  to  the  car  as  if  the  latter  were  at 
rest.  But  if  we  refer  it  to  the  surface  of  the  earth,  lie  would 
be  going  east  at  the  rate  of  47  miles  an  hour.  Hence,  in 
speaking  of  the  motion  of  a  body,  there  must  always  be  st>me 
other  body,  or  some  position,  to  which  the  motion  is  referred. 

In  everyday  life  we  commonly  refer  the  motions  of  things 
around  us  to  the  surface  of  the  earth.  In  astronomy  motions 
are  sometimes  referred  to  the  center  of  the  earth,  or  to  the  sun, 
or  even  to  the  stars. 

3.  The  Earth.  —  Some  of  the  following  facts  are  taught  in 
geography,  but  they  are  of  equal  importance  in  astronomy  :  — 

1.  The  earth  has  the  form  of  a  spheroid.     Its  figure  is  so 
near  that  of  a  globe  that  the  eye  could  not  see  any  deviation 
from  the  spherical  form.     Hence,  we  commonly  speak  of  the 
earth  as  a  globe. 

2.  We  live  on  the  round  surface  of  this  globe. 

3."  Our  bodies  and  everything  else  on  the  earth's  surface  are 
drawn  toward  its  center  by  a  force  called  gravity.  Were  it 


RELATION  OF  THE  EARTS  TO   THE  HEAVENS      11 

not  for  gravity,  objects  on  the  earth  would  have  no  tendency  to 
stay  there. 

4.  It  follows  that  the  direction  we  call  downward  is  not  the 
same  in  any  two  places,  because  it  is  everywhere  nearly  toward 
the  earth's  center.     Dwellers  on  the  opposite  side  of  the  earth 
stand  with  their  feet  pointing  toward  us,  and  are  therefore 
called  our  antipodes. 

5.  The  earth  turns  continually  from  west  to  east  on  an 
imaginary  line  passing  through  its  center,  and  called  its  axis. 
The  two  opposite  points  in  which  the  axis  intersects  the  surface 
of  the  earth  are  called  poles.     One  of  these  is  called  the  north 
pole,  the  other  the  south  pole.     The  time  required  to  make  a 
revolution  is  called  a  day. 

6.  An  imaginary  circle  passing  round  the  earth,  equally  dis- 
tant from  the  two  poles,  is  called  the  earth's  equator. 

7.  The  motion  of  the  earth  on  its  axis  is  so  smooth  and  uni- 
form that  we  are  entirely  unconscious  of  it.     Hence  it  seems 
to  us  to  be  at  rest  while  the  heavenly  bodies  seem  to  move  in 
the  opposite  direction,  from  east  toward  west. 

4.  The  Celestial  Sphere. — When  we  look  up  from  the  earth, 
the  stars  seem  to  be  set  in  a  blue  vault  or  dome,  which  we  call 
the  sky.  The  sky  seems  to  rise  high  over  our  heads,  and  to 
curve  down  on  every  side  toward  the  earth,  on  which  it  seems 
to  rest.  The  sky  is  not  a  real  object,  but  only  an  appearance 
produced  by  the  blue  light  reflected  from  the  air  to  our  eyes. 

There  are  as  many  stars  in  the  heavens  by  day  as  by  night. 
The  reason  we  do  not  see  them  by  day  is  that  our  eyes  are 
dazzled  by  the  light  of  the  sky,  which  is  really  light  reflected 
by  the  air.  If  we  could  mount  above  the  air,  we  should  see  no 
sky,  because  there  would  be  no  air  to  reflect  the  light,  and  we 
should  see  the  stars  all  day  as  well  as  all  night. 

The  stars  surround  us  in  every  possible  direction,  below  our 
feet  as  well  as  above  our  heads.  The  earth  is  in  the  way  of 
our  seeing  them  when  they  are  below  us,  but  they  are  then  visi- 
ble to  our  antipodes. 


12 


ASTRONOMY 


The  heavenly  bodies  are  really  at  very  different  distances. 
They  appear  to  us  to  be  at  the  same  distance  because  our  eyes 
cannot  distinguish  their  distances  as  less  or  greater.  Hence 
we  fancy  them  to  be  on  the  surface  of  a  hollow  sphere,  in  the 

Q 


FIG.  1.  —  Showing  how  stars p,  7,  r,  s,  etc.,  are  seen  by  an  observer  at  0 
as  if  they  all  lay  on  a  sphere  at  the  respective  points  P,  @,  J?,  S, 
etc.  The  three  stars  marked  t  are  seen  as  if  they  were  a  single  star 
in  the  position  T,  because,  being  in  the  same  straight  line  from  the 
observer,  they  cannot  be  distinguished. 

center  of  which  we  stand.  Although  this  sphere  is  imaginary, 
it  will  help  our  thoughts  to  think  of  it  and  talk  about  it  as  if 
it  were  real.  It  is  called  the  celestial  sphere. 

We  must  imagine  the  celestial  sphere  to  be  so  vast  that  the 
earth  in  its  center  is  a  mere  point  in  comparison. 

5.  Perspective  of  Plane  and  Line.  —  You  doubtless  know  that 
a  plane  is  a  flat  surface  which  may  be  supposed  to  extend  out 
all  round  us  as  far  as  we  please.  When  we  represent  a  plane 
on  paper  we  have  to  give  it  a  boundary  because  there  is  not 
room  enough  on  the  paper  to  show  it  extending  out  without 


RELATION  OF  THE  EARTH  TO   THE  HEAVENS      13 


limit.  We  are  not  to  suppose  that  the  circle  or  other  bound- 
ing line  on  the  figure  is  really  the  boundary  of  the  plane ;  the 
latter  need  not  have  a  boundary. 

Let  us  see  how  we  may  represent  a 
plane  seen  in  different  ways.  Figure 
2,  a,  shows  a  small  portion  of  a  plane, 
bounded  by  a  circle,  on  which  we  are 
looking  perpendicularly.  This  plane 
coincides  with  the  plane  of  the  paper. 

Figure  2,  6,  shows  the  plane  seen 
obliquely.  We  must  conceive  this 
plane  as  passing  through  the  plane 
of  the  paper. 

Figure  2,  c,  shows  the  same  plane 
seen  edgewise.  It  then  looks  like  a 
straight  line,  but  must  still  be  con- 
ceived as  a  plane,  and  as  being  per- 
pendicular to  the  plane  of  the  paper. 


6.  Angular  Measure  on  the  Celestial  FIG.  2. 

Sphere.  —  When  we  speak  of  the  dis- 
tance of  two  heavenly  bodies  from  each  other,  the  word  dis- 
tance may  have  either  of  two  meanings. 

The  real  distance  is  the  length  of  the  line  from  one  body  to 
the  other.  Hence  this  is  also  called  linear  distance. 

The  apparent  distance  between  two  heavenly  bodies  is  their 
distance  apart  as  it  appears  to  us.  This  is  not  a  line,  but  the 
angle  between  two  lines  from  the  observer's  eye,  one  going 
toward  one  body  and  one  toward  the  other.  In  astronomy  we 
commonly  use  words  expressing  distance  in  this  sense;  thus 
we  say  that  the  moon  and  a  star  are  together,  or  that  a  star  is 
alongside  the  moon,  when  they  look  so  to  our  eyes,  although 
the  star  is  in  reality  millions  of  times  farther  from  us  than  the 
moon.  Two  heavenly  bodies  appear  together  when  they  lie  in 
the  same  line  from  the  observer. 

The  apparent  distance  being  an  angle,  is  measured  as  angles 


14 


ASTRONOMY 


are  measured  in  other  cases,  the  position  of  the  observer  being 
the  vertex  of  the  angle.  Imagine  a  circle  on  the  celestial 
sphere  with  the  eye  of  the  observer  in  the  center  as  shown  in 
figure  3. 


FIG.  3.  —  Showing  the  angle  between  two  stars,  a  and  6,  as  seen  by  an 
observer.  In  the  figure  this  angle  is  about  10°.  The  figure  also  shows 
how  degrees  are  counted  round  the  circle,  passing  from  the  line  going 
horizontally  to  the  right.  A  right  angle,  or  90°,  measures  from  the 
horizon  A  to  the  zenith  B.  Two  right  angles,  or  180°,  bring  us  to 
the  horizon  on  the  left ;  the  third  right  angle,  making  270°,  takes  us 
to  the  point  D  below,  and  the  fourth  one  will  carry  us  round  to  A, 
where  we  started. 

Then  this  circle  is  divided  into  four  arcs,  AB,  BC,  CD, 
and  DA,  each  of  which  measures  a  right  angle  at  the  center. 


RELATION  OF  THE  EARTH  TO   THE  HEAVENS      15 

The  right  angle  is  subdivided  into  90°,  which  may  be  done  by 
dividing  the  arcs  AB,  etc.,  into  90  equal  parts.  Each  degree  is 
divided  into  60  minutes,  and  each  minute  into  60  seconds. 

Any  little  arc  on  the  sphere  is  then  said  to  subtend  the 
angle  formed  between  the  lines  drawn  from  the  observer's  eye 
to  the  ends  of  the  arc. 

To  give  an  idea  of  the  magnitude  of  angles,  a  foot  rule  at 
the  distance  of  57  feet  from  the  eye  subtends  an  angle  of 
about  1°. 

The  diameters  of  the  sun  and  moon  subtend  an  angle  of  a 
little  more  than  half  a  degree. 

The  diameter  of  the  smallest  round  object  that  an  ordinary 
eye  can  distinctly  see  subtends  an  angle  of  1'.  More  exactly 
V  is  the  angle  subtended  by  a  nickel  at  a  distance  of  320  feet. 

7.   The  Relation  of  the  Horizon  to  the  Celestial  Sphere.  —  In 

ordinary  language  the  line  around  us  where  the  earth  and 
sky  seem  to  meet  is  called  the  visible  horizon,  or  simply  the 
horizon.  On  a  ship  at  sea  the  visible  horizon  is  a  circle, 
extending  all  round  the  observer.  It  is  called  the  sea  horizon. 


FIG.  4.  — The  dip  of  the  horizon  from  the  deck  of  a  ship.  The  curve  is 
the  rounded  ocean  ;  ST  is  a  horizontal  line  which  does  not  strike  the 
ocean  at  all ;  H  is  a  point  of  the  sea  horizon,  seen  from  the  ship  in 
the  line  SH.  The  angle  T8H  between  the  horizontal  line  from  the 
observer's  eye  and  the  sea  horizon  is  the  dip  of  the  horizon. 

If  we  regard  the  earth  as  perfectly  round  and  smooth,  a 
plane  resting  on  it  at  a  point  where  we  stand  is  called  the 
plane  of  the  horizon,  or  the  horizon  plane.  As  we  look  around, 
we  must  imagine  this  plane  to  extend  out  indefinitely  on  all 
sides  of  us. 

In  figure  4  we  show  the  relation  of  the  horizon  plane  to  the 


16 


ASTRONOMY 


sea  horizon.  The  observer's  eye  being  higher  than  the  water, 
we  see  from  this  figure  that  the  sea  horizon  will  appear  a 
little  below  the  horizon  plane.  The  angle  by  which  it  seems 
below  is  called  the  dip  of  the  horizon.  The  higher  the  eye  is 
above  the  sea,  the  greater  the  dip, 


FIG.  5.  —  Showing  the  altitude  of  a  body  above  the  horizon.  It  is  the 
angle  between  the  body  and  the  horizon  H  as  it  appears  to  an  ob- 
server. The  zenith  distance  is  the  angle  between  the  line,  in  which 
the  body  is  seen,  and  the  zenith  Z. 

The  point  in  the  heavens  over  our  head  (B,  fig.  3)  is  called 
the  zenith  ;  the  point  in  the  celestial  sphere  below  our  feet 
(D9  fig.  3),  which  we  cannot  see,  is  called  the  nadir. 

The  altitude  of  a  heavenly  body  is  the  angle  which  its  direc- 


RELATION   OF  THE  EARTH  TO   THE  HEAVENS      17 

tion  makes  with  the  plane  of  the  horizon.  The  greater  its 
altitude,  the  higher  it  seems  to  ns  to  be. 

The  line  joining  the  zenith  or  nadir  to  the  point  where  we 
stand  is  called  a  vertical  line.  It  is  evident  that  such  a  line  is 
perpendicular  to  the  horizon  plane.  Its  direction  is  that  of 
the  plumbline. 

The  zenith  distance  of  a  body  is  its  apparent  angular  distance 
from  the  zenith. 

Zenith  distance  and  altitude  added  together  make  up  the 
whole  arc  from  the  zenith  to  the  horizon,  which  measures 
90°. 

Change  of  Horizon  as  we  Travel.  —  Now  let  us  conceive  the 
celestial  sphere  with  the  earth  in  its  center.  Let  the  globe  in 
figure  6  represent  the  earth,  and  let  APE  be  the  horizon  plane 
of  an  observer  standing  at  P.  We  imagine  this  plane  extend- 
ing out  all  round  until  it  meets  the  celestial  sphere.  We  can- 
not draw  this  sphere  round  figure  6  because,  to  be  large  enough 
in  proportion  to  the  earth,  we  should  have  to  make  it  bigger 
than  a  house.  So  we  draw  it  on  a  smaller  scale  in  figure  7. 
On  this  scale  the  earth  is  a  mere  point  in  the  center. 

The  plane  of  the  horizon  at  P  in  figure  6  cuts  the  celestial 
sphere  in  a  circle  AB,  seen  edgewise  in  figure  7.  This  circle 
is  called  the  celestial  horizon  of  the  observer  at  P  in  figure  6. 
We  must  imagine  it  extending  all  around  us,  like  the  visible 
horizon.  The  celestial  horizon  divides  the  celestial  sphere 
into  two  hemispheres,  ACB  and  BDA. 

Standing  at  P  in  figure  6,  we  can  see  the  stars  in  the  hemi- 
sphere ACB,  figure  7,  which  is  therefore  called  the  visible 
hemisphere.  We  cannot  see  the  stars  in  the  hemisphere  ADB 
because  the  earth  is  in  the  way,  so  this  is  called  the  invisible 
hemisphere. 

We  see  in  figures  6  and  7  that,  as  we  travel  from  one  place 
to  another,  the  horizon  changes  its  direction,  so  that  stars,  be- 
fore invisible,  will  come  into  view  on  one  side,  and  visible  ones 
will  seem  to  sink  below  the  horizon  on  the  other  side.  For 
example,  if  the  observer  travels  to  the  point  Q  (fig.  6),  his 
NEWCOMB'S  ASTRON. —  2 


18 


ASTRONOMY 


horizon  plane  will  be  CD,  in  figure  7.  Then  the  stars  in  the 
region  N,  between  B  and  D,  will  come  into  view,  and  those  in 
the  region  M,  between  A  and  (7,  will  seem  to  sink  below  the 
horizon. 


B 


FIG.  6. — Showing  how  the  horizon  changes  as  an  observer  travels  from  one 
point  of  the  earth  to  another.  The  straight  lines  AB  and  CD  touch- 
ing the  earth  represent  planes  seen  edgewise.  These  are  planes  of  the 
horizon,  or  horizon  planes.  Every  different  point  of  the  earth's  sur- 
face has  a  different  horizon  plane.  For  example  :  when  the  observer 
is  at  the  point  P,  his  horizon  plane  will  be  AB,  which  is  represented 
as  a  line  because  we  see  it  edgewise.  If  he  travels  to  the  point  $,  his 
horizon  plane  will  turn  round  into  the  position  CD.  Thus  his  horizon 
always  changes  as.  he  travels.  These  horizon  planes  must  be  supposed 
extended  out  on  all  sides  until  they  cut  the  celestial  sphere.  As  the 
scale  in  this  figure  is  too  large  to  represent  the  celestial  sphere,  we 
make  another  figure  on  a  much  smaller  scale  to  show  the  continuation 
of  the  horizon  planes. 

It  is  very  interesting  to  watch  this  change  during  a  long 
ocean  voyage  from  the  north  to  the  south,  or  vice  versa.  As 
we  steam  -along  the  rounded  surface  of  the  ocean,  we  see  the 
constellations  behind  us  sinking  lower  and  lower  every  night, 
till  they  disappear  below  the  ocean  horizon,  while  new  ones 


RELATION  OF  THE  EARTH   TO   THE  HEAVENS      19 

seem  to  rise  from  the  ocean  in  front  of  us.     Owing  to  the  same 
cause  the  days  are  more  than  24  hours  long  when  we  cross  the 


FIG.  7.  —The  celestial  sphere,  with  the  earth  a  mere  point  in  the  center 
at  E,  showing  two  horizontal  planes  corresponding  to  those  of  figure 
6,  extended  until  they  intersect  the  celestial  sphere.  As  an  observer 
travels  from  P  to  Q,  in  figure  6,  his  horizon  planes  change  from  the 
position  AS  to  the  position  CD,  in  figure  7.  Thus  the  stars  in  the 
region  Jf,  which  are  visible  when  the  observer  is  at  P,  are  invisible 
when  he  is  at  Q,  while  the  reverse  is  true  in  the  region  N.  The  posi- 
tion of  the  zenith  on  the  celestial  sphere  also  changes  from  Z\  to  Z2, 
as  the  observer  travels  from  P  to  Q. 

ocean  from  Europe  to  America,  and  less  than  24  hours  long 
when  we  cross  from  America  to  Europe. 


20  ASTRONOMY 

8.  The  Diurnal  Motion.  —  If,  in  our  latitudes,  we  look  at  any 
star  south  of  the  zenith,  and  watch  it  for  an  hour,  we  shall  find 
it  moving  from  the  left  toward  the  right.    If  we  watch  it  a  few- 
hours,   we  shall   see  that  it  moves  yet  farther   and   farther 
toward  the  right  and  at  length  sets  in  the  west,  like  the  sun. 
If  we  look  at  a  star  in  the  east,  we  shall  find  it  rising  upward 
and  moving  toward  the  south  as  the  sun  does  every  day.     Thus 
the  stars  in  the  south  rise  and  set  like  the  sun. 

We  know  that  this  is  because  the  earth  turns  on  its  axis. 
To  us,  the  appearance  is  the  same  whether  we  suppose  the 
earth  to  turn  and  the  stars  to  stand  still,  which  is  the  truth,  or 
whether  we  suppose  the  earth  to  be  still  and  the  stars  to  re- 
volve round  it,  which  is  not  the  truth.  In  order  to  describe 
how  things  look,  it  is  sometimes  easier  to  suppose  that  the 
earth  is  still,  as  we  may  fancy  it  to  be,  and  then  to  tell  how 
the  stars  seem  to  move. 

We  call  any  seeming  motion  of  the  heavenly  bodies,  sun, 
moon,  and  stars,  their  apparent  motion,  which  means  their 
motion  as  it  appears  to  us. 

The  apparent  motion  which  they  have  in  consequence  of  the 
earth  turning  on  its  axis,  is  called  the  diurnal  motion.  Hence, 
the  motion  by  which  the  sun,  moon,  and  stars  rise  and  set 
every  day  is  the  diurnal  motion. 

9.  Celestial  Equator  and  Poles.  —  Figure  8  shows  the  earth 
with  its  axis  passing  through  its  center,  from  the  south  to  the 
north  pole.     We  must  imagine  this  axis  continued  as  far  as 
we  please  in  both  directions. 

Imagine  a  plane  EQ  passing  through  the  center  of  the 
earth,  at  right  angles  to  its  axis.  This  is  called  the  plane  of 
the  equator,  because  it  intersects  the  earth's  surface  all  round 
on  the  equator. 

We  must  conceive  this  plane  to  be  extended  out  as  far  as* we 
please.  Now  we  draw  figure  9  on  a  small  scale.  The  outer 
circle  represents  the  imaginary  celestial  sphere  with  the  earth 
as  a  black  dot  in  the  center. 


OF  THE  EARTH  TO  THE  HEAVENS   21 


the  points  NP  and  SP,  in  which  the  line  of  the  earth's  axis 
intersects  the  celestial  sphere,  are  called  the  celestial  poles. 

The  one  which  is  visible  to  us,  NP,  is  called  the  north 
celestial  pole;  the  other,  /SP,  which  is  below  our  horizon,  is 
the  south  celestial  pole. 

The  plane  of  the  equator  continued  out  in  every  direction 
intersects  the  celestial  sphere  in  a  circle  EQ,  which  is  called 


FIG.  8.  —  Showing  the  plane  of  the  earth's  equator.  The  equator  itself 
passes  around  the  earth  and  cuts  the  earth  into  two  equal  hemi- 
spheres. The  plane  of  the  equator  extends  outward  on  all  sides  until 
it  meets  the  celestial  sphere.  In  the  figure  we  have  to  show  it  bounded 
by  a  circle  because  there  is  no  room  to  represent  it  going  any  farther, 
but  in  reality  it  has  no  boundary. 

the  celestial  equator.  The  celestial  equator  is  an  imaginary 
circle  of  the  celestial  sphere  which  always  has  the  same  posi- 
tion in  the  heavens.  It  intersects  the  horizon  in  the  east  and 
west  points,  and,  in  northern  latitudes,  passes  south  of  the 
zenith  by  a  distance  equal  to  the  latitude  of  the  place.  One 
half  of  it  is  above,  the  other  half  below,  the  horizon. 


22  ASTRONOMY 

10.  The  Meridian.  —  A  line  on  the  earth's  surface,  from  the 
north  to  the  south  pole,  is  called  a  terrestrial  meridian  or  simply 
a  meridian.  Owing  to  the  curvature  of  the  earth  its  form  is 
that  of  a  semicircle. 


FIG.  9.  — The  celestial  sphere,  with  the  earth  a  point  in  the  center  at  0, 
showing  how  some  stars  rise  and  set,  while  others  never  set,  and 
others,  in  our  latitudes,  never  rise.  The  circle  of  perpetual  appari- 
tion, within  which  the  stare  never  set,  is  round  the  north  celestial 
pole,  which  is  above  our  horizon.  Those  which  rise  and  set  are  on 
both  sides  of  the  celestial  equator. 

The  figure  shows  how  the  celestial  meridian  SENP  passes  over  the 
celestial  sphere  in  a  north  and  south  direction  through  the  zenith. 

Such  a  line  may  pass  through  any  place :  it  is  then  called 
the  meridian  of  that  place.  Thus  we  speak  of  the  meridian  of 
Greenwich,  Washington,  or  Chicago,  meaning  the  semicircles 
joining  either  of  these  points  to  the  poles  of  the  earth. 


RELATION  OF  THE  EARTH  TO  THE  HEAVENS   23 

The  north  and  south  direction  is  that  of  the  meridian.  Any 
number  of  places  may  be  on  the  same  meridian ;  they  are  then 
north  and  south  of  each  other. 

The  plane  of  the  meridian  of  any  place  is  a  plane  passing 
through  the  place  and  the  north  and  south  poles. 

If  we  imagine  this  plane  extended  upward,  so  as  to  intersect 
the  celestial  sphere,  the  circle  of  intersection  is  called  the 
celestial  meridian.  The  celestial  meridian  passes  through  the 
celestial  poles  and  the  zenith  of  the  place. 

In  figure  9  the  plane  of  the  paper  is  the  plane  of  the  merid- 
ian, and  the  outer  circle  of  the  figure  is  the  celestial  meridian. 
The  points  N  and  8  in  which  it  intersects  the  plane  of  the 
horizon  are  the  north  and  south  points  of  the  horizon.  Hence 
if  one  stands  facing  the  south,  his  meridian  rises  perpen- 
dicularly from  the  south  horizon  to  the  zenith  and  continues 
to  the  celestial  pole. 

11.  Diurnal  Motion  in  Different  Latitudes. — The  apparent 
diurnal  motion  takes  place  as  if  the  two  celestial  poles  were 
pivots  on  which  the  celestial  sphere  is  continually  turning. 

Suppose  a  person  to  stand  at  the  north  pole  of  the  earth. 
Then  the  celestial  sphere  will  appear  to  him  as  it  is  represented 
in  figure  10.  The  north  celestial  pole  will  be  over  his  head, 
that  is,  in  his  zenith.  The  plane  of  his  horizon  will  be  MN, 
which  you  see  is  parallel  to  the  plane  of  the  equator.  When 
the  earth  becomes  a  mere  point,  these  planes  are  so  close 
together  that  we  cannot  distinguish  between  them.  Hence :  — 

To  an  observer  at  the  north  pole,  the  north  celestial  pole  is  at  the 
zenith,  the  celestial  equator  is  in  the  horizon,  and  the  south  pole  is 
at  his  nadir. 

The  poles  being  pivots,  the  diurnal  motion  of  each  heavenly 
body  will  seem  to  go  on  in  a  horizontal  circle,  from  left  to 
right,  so  that  none  of  these  bodies  will  either  rise  or  set. 
When  the  observer  is  near  the  pole,  the  motion  will  be  nearly 
horizontal. 


24 


ASTRONOMY 


Suppose  the  observer  travels  to  latitude  40  degrees,  which  is 
nearly  that  of  New  York  and  Philadelphia.  We  have  seen  in 
figures  6  and  7  how  his  horizon  plane  turns  round  as  he  travels. 
As  it  turns,  the  celestial  pole  will  appear  to  move  over  to  a 
position  where  it  will  be  40  degrees  above  the  horizon  and  50 
degrees  from  the  zenith. 


Nadir 


FIG.  10. — Showing  the  equator,  horizon,  and  direction  of  the  diurnal 
motion  as  seen  by  an  observer  at  the  north  pole  of  the  earth.  The 
earth  is  an  invisible  point  in  the  center  at  E.  The  outer  circle  is 
the  celestial  sphere  as  seen  by  the  observer.  Round  his  horizon  on 
the  celestial  sphere  is  the  celestial  equator,  which  in  this  particular 
case  is  the  same  as  the  celestial  horizon.  The  other  horizontal  circles 
show  the  diurnal  motion  of  the  heavenly  bodies,  which  seem  to 
make  their  revolutions  round  and  round  in  horizontal  circles,  without 
either  rising  or  setting. 

In  this  position  the  northern  heavens  appear  as  in  figure  11. 
The  celestial  pole  is  marked  in  the  center  of  the  map.  There 
is  a  fairly  bright  star  so  near  the  pole  that  it  is  called  the  Pole- 
star.  If  one  has  an  exact  north  and  south  line,  he  can  find 
the  polestar  by  looking  nearly  halfway  between  the  horizon 
and  the  zenith,  toward  the  north.  Mf  not,  he  must  find  the 


RELATION  OF  THE  EARTH  TO   THE  HEAVENS      25 

constellation  Ursa  Majorj  commonly  called  the  Dipper.  The 
two  stars  which  form  the  outside  of  the  dish  of  the  Dipper 
point  very  nearly  at  the  polestar,  as  we  see  by  the  dotted  line 


FIG.  11. — The  circle  of  perpetual  apparition  to  an  observer  in  latitude 
40°,  showing  the  brightest  stars  of  the  northern  constellations,  includ- 
ing the  pointers  in  Ursa  Major  pointing  at  the  north  star.  To  see 
how  the  stars  will  look  at  half-past  eight  o'clock  on  any  evening  in 
the  year,  hold  the  figure  with  the  month  at  the  bottom  and  look  at 
the  stars  at  the  corresponding  hour.  The  hour  given  is  for  the 
middle  of  the  month.  To  find  the  positions  at  other  hours,  notice 
that  the  diurnal  motion  takes  place  in  a  direction  the  opposite  of 
those  of  the  hands  of  a  clock,  as  shown  by  the  arrows,  and  turn  the 
figure  accordingly. 

in  the  figure.     The  Dipper  can  be  seen  almost  any  evening  in 
the  year,  but  in  the  evenings  of  autumn  it  will  be  low  down 


26 


ASTRONOMY 


near  the  northern  horizon.  If  the  pointers  cannot  be  seen, 
there  is  only  one  other  star  that  there  is  danger  of  mistaking 
for  the  polestar.  This  is  Beta  Ursce  Minoris,  or  Beta  of  the 
Little  Bear,  which  is  about  15  degrees  from  it  and  is  of  the 
same  brightness ;  but  it  can  still  be  distinguished  by  its  being 
a  little  redder  and  by  the  two  stars  between  which  it  is 
situated. 

Having  found  the  polestar,  imagine  a  circle  to  be  drawn  in 
the  heavens,  having  the  pole  as  a  center,  and  at  such  a  distance 


N.R 


as  to  graze  the  northern  horizon.  Then  studying  figure  11,  yon 
will  see  that,  as  the  celestial  sphere  appears  to  revolve  around 
the  pole  as  a  pivot,  all  the  stars  within  this  circle  will  merely 
turn  round  and  round  the  pole,  as  shown  by  the  arrows,  but 
none  of  them  will  ever  rise  and  set.  Hence  this  circle  is 
called  the  circle  of  perpetual  apparition. 

It  will  readily  be  seen  that  the  stars  but  a  little  outside  this 
circle  dip  only  a  short  distance  below  the  horizon  and  are  but 
a  short  time  below  it.  They  are  above  the  horizon  most  of  the 


RELATION  OF  TEE  EARTH  TO  THE  HEAVENS      27 

time,  but  not  all  the  time.     The  farther  they  are  from  this  cir- 
cle, the  longer  they  are  below  the  horizon. 

Next  let  us  study  figure  12.  This  shows  the  celestial  sphere 
as  if  we  were  looking  at  it  from  the  east,  so  that  we  see  the 
western  portion  of  it.  Here  you  see  that  the  equator  EQ  is 
one  half  above  the  horizon  HR,  and  one  half  below  it.  Hence 


South 


North 


FIG.  13.  —  Showing  the  diurnal  motion  as  seen  by  an  observer  at  the 
equator.  The  earth  is  a  point  in  the  center  at  E.  The  celestial  poles 
are  in  the  north  and  south  horizon,  and  the  diurnal  motion  takes 
place  in  vertical  circles  shown  in  the  figure.  All  the  heavenly  bodies 
are  as  long  above  the  horizon  as  below  it,  except  for  a  small  effect  of 
refraction,  which  will  be  hereafter  explained. 

a  star  on  the  equator  is  12  hours  above  the  horizon  and  12 
hours  below  it,  in  the  course  of  each  apparent  diurnal  revo- 
lution. 

The  farther  south  the  star  is  situated,  the  shorter  the  time  it 
is  above  the  horizon  and  the  longer  the  time  it  is  below  it.     At 


28  ASTRONOMY 

50  degrees  south  of  the  equator,  the  star  will  barely  appear  on 
the  horizon  and  will  immediately  sink  below  it  again. 

Now  notice  the  south  celestial  pole,  as  shown  in  figure  9. 
We  see  that,  as  the  sphere  seems  to  turn  on  it  as  &  pivot,  if  we 
imagine  a  circle  drawn  so  as  to  touch  our  southern  horizon,  the 
stars  within  this  circle  will  never  seem  to  us  to  rise  at  our 
latitude.  Hence  this  circle  is  called  the  circle  of  perpetual 
occultation. 

If  we  travel  yet  farther  south,  say  to  the  Gulf  of  Mexico, 
the  north  celestial  pole  being  nearer  to  the  horizon,  the  circle 
of  perpetual  apparition  will  be  smaller.  The  circle  of  perpet- 
ual disappearance  will  also  be  smaller. 

If  we  travel  to  the  equator,  the  two  poles,  or  seeming  pivots, 
being  in  the  north  and  south  horizon,  all  the  stars  will  rise  and 
set,  each  being  as  long  above  the  horizon  as  below  it.  Hence 
there  will  be  no  circles  of  perpetual  apparition  or  occultation. 

If  we  go  on  into  the  southern  hemisphere,  the  south  celestial 
pole  will  rise  above  our  horizon,  the  circle  of  perpetual  appari- 
tion will  be  around  it,  and  that  of  perpetual  disappearance  will 
be  round  the  north  pole,  now  below  the  horizon. 

Because  a  meridian  line  is  fixed  on  the  earth's  surface,  it 
follows  that  all  the  meridians  revolve  with  the  earth.  Hence 
one  result  of  the  diurnal  motion  is  that  all  the  heavenly 
bodies  seem  to  pass  the  meridian  every  day.  Really,  the 
meridian  passes  them,  but,  to  our  senses,  they  seem  to  pass 
the  meridian. 

12.  Right  Ascension  and  Declination.  —  The  Decimation  of  a 
heavenly  body  is  its  apparent  distance  from  the  celestial 
equator.  To  measure  it  we  imagine  a  circle  to  pass  from  the 
pole  through  the  body  S,  figure  14,  to  the  celestial  equator. 
This  is  called  an  hour  circle.  The  arc  SR  of  this  circle,  be- 
tween S  and  the  equator,  is  the  declination  of  the  body  S. 
It  is  measured  in  degrees,  minutes,  and  seconds. 

When  the  body  is  north  of  the  equator,  as  at  S,  it.  is  said  to 
be  in  North  Declination;  when  south,  in  South  Declination. 


RELATION   OF  THE  EARTH  TO   THE  HEAVENS      29 

Comparing  figures  14  and  9,  we  shall  see  that  if  we  are  in 

north  latitude,  the  farther  north  the  declination  of  a  body, 

the  longer  it  will  be  above  our  horizon  during  its  diurnal 
revolution. 


North 

Pole 


FIG.  14.  —  Showing  the  hour  circles  passing  from  one  celestial  pole  to  the 
other,  and  the  right  ascension  and  declination  of  a  star.  The  first 
hour  circle  is  represented  as  passing  to  the  left  of  the  earth  through 
V.  The  declination  of  the  star  8  is  its  distance  from  the  equator  7?, 
and,  in  the  figure,  is  about  23°.  Its  right  ascension  is  the  arc  from 
V  to  R  on  the  celestial  sphere,  which  is  here  three  hours. 

The  right  ascension  of  a  heavenly  body  is  the  angle  which 
the  hour  circle  drawn  through  it  makes  with  that  through  the 
vernal  equinox  Y\  a  point  to  be  defined  hereafter. 

Astronomers  commonly  measure  right  ascension,  like  time, 
in  hours,  minutes,  and  seconds.  As  the  earth  turns  through 
360°  in  24  hours,  it  turns  15°  in  every  hour. 


80  ASTRONOMY 

The  position  of  a  point  on  the  celestial  sphere  —  a  star,  for 
example  —  is  expressed  by  its  right  ascension  and  declination, 
much  as  the  position  of  a  city  on  the  earth's  surface  is 
expressed  by  its  longitude  and  latitude.  To  understand  right 
ascension  we  recall  that  the  longitude  of  a  place  on  the  earth's 
surface  is  the  angle  between  two  terrestrial  meridians,  one  of 
which  passes  through  the  Royal  Observatory,  Greenwich,  and 
the  other  through  the  place.  On  maps  are  drawn  as  many 
meridians  as  are  necessary,  each  being  numbered  according  to 
its  angle  with  the  Greenwich  meridian.  The  longitude  of  a 
place  on  the  map  is  learned  by  its  position  relatively  to  the 
meridian  lines  which  pass  on  each  side  of  it. 

On  the  same  plan,  astronomers  suppose  semicircles  to  pass 
on  the  celestial  sphere  from  one  pole  to  the  other,  as  shown  in 
figure  14.  These  circles  are  really  celestial  meridians.  But  the 
latter  are  supposed  to  move  with  the  revolving  earth,  while  the 
semicircles  we  speak  of,  that  is,  the  hour  circles,  are  fixed  on 
the  celestial  sphere. 

The  first  hour  circle,  taken  as  a  standard,  like  the  meridian 
of  Greenwich,  is  that  which  passes  through  the  vernal  equinox. 
In  figure  14  the  vernal  equinox  is  at  V. 

13.  Correspondence  of  the  terrestrial  and  celestial  spheres.  —  On 
the  earth  a  parallel  of  latitude  is  a  circle  parallel  to  the  equator. 
All  points  on  it  have  the  same  latitude. 

In  the  heavens  a  parallel  of  declination  is  a  circle  parallel  to 
the  celestial  equator.  All  points  on  it  have  the  same  declination. 

The  zenith  of  every  point  on  the  earth's  surface  lies  on  the 
corresponding  parallel  of  declination.  That  is,  if  you  are  in 
40°  of  north  latitude,  a  star  exactly  over  your  head  will  be  in 
40°  of  declination.  Thus,  as  shown  in  figure  14,  every  parallel 
of  declination  passes  vertically  over  the  corresponding  parallel 
of  latitude. 

At  any  point  on  the  earth's  surface  the  altitude  of  the  celestial  pole 
is  equal  to  the  latitude  of  the  place. 

The  arc  of  the  meridian  from  the  zenith  to  the  celestial  equator  is 
also  equal  to  the  latitude  of  the  place. 


RELATION  OF  THE  EARTH  TO   THE  HEAVENS      31 

To  see  this,  suppose  yourself  standing  at  the  equator.  Then 
the  celestial  equator,  as  already  explained,  passes  through  your 
zenith,  and  the  pole  is  in"  your  horizon. 

Now  travel  north  through  1°  of  latitude.  Then  your  horizon 
will  have  tipped  1°  below  the  celestial  pole,  and  your  zenith 
will  have  moved  1°  from  the  celestial  equator.  The  correspond- 
ing result  will  occur  how  far  soever  you  travel.  In  latitude 
40°  your  horizon  will  have  tipped  40°  below  the  pole,  so  that 
the  latter  is  now  40°  above  the  horizon,  and  your  zenith  will 
have  moved  40°  from  the  equator.  In  latitude  45°  the  pole 
will  be  midway  between  the  zenith  and  the  north  horizon ; 
the  equator  will  be  midway  between  the  zenith  and  the  south 
horizon. 

Algebraic  Expression  of  the  Relation  between  Declination,  Zenith 
Distance,  and  Latitude.  —  Algebraic  symbols  are  used  to  express 
these  three  quantities,  the  following  notation  being  used :  — 

L  =  latitude,  +  when  north ;   —  when  south. 

D  =  declination  ;  -f  when  north  ;   —  when  south. 

Z  —  zenith  distance ;  +  when  south ;   —  when  north. 

If  a  star  on  the  meridian  is  a  certain  distance  Z  south  of  the 
zenith,  its  declination  will  be  less  than  that  of  the  zenith  by  Z. 
At  the  zenith  we  have  Z  =  0  and  L  =  D.  Hence  we  shall 
always  have  D  =  L  —  Z,  or  L  =  D  +  Z. 

Right  ascension  on  the  celestial  sphere  and  longitude  on  the 
earth  would  correspond  in  the  same  way,  were  it  not  that  the 
rotation  of  the  earth  keeps  each  meridian  in  constant  motion 
over  the  hour  circles  of  the  celestial  sphere.  But  every  day 
there  is  a  certain  moment  at  which  the  vernal  equinox  passes 
over  the  meridian  of  Greenwich.  At  this  moment  the  right 
ascension  of  a  star  on  the  meridian  of  any  place  is  equal  to  the 
east  longitude  of  the  place  from  Greenwich.  Hence,  at  this 
particular  moment  there  is  a  correspondence  of  the  meridians 
on  the  earth  and  in  the  heavens. 


CHAPTER   II 


THE  REVOLUTION  OF  THE   EARTH  ROUND  THE  SUN 

1.  The  Earth  as  a  Planet.  —  In  the  preceding  chapter  we 
have  explained  the  various  phenomena  which  arise  from  the 
rotation  of  the  earth  on  its  axis.  We  have  to  explain  another 
change  with  which  we  are  all  familiar,  —  that  of  the  seasons. 
We  know  that  these  go  through  a  regular  change  in  a  period 
of  about  365  days.  During  one  part  of  this  period,  which  we 
call  summer,  we  see  the  sun  rise  to  the  north  of  east,  pass  the 
meridian  high  up  in  the  heavens,  and  set  to  the  north  of  west. 
At  the  opposite  season  the  sun  rises  south  of  east,  culminates 
low  in  the  south,  and  sets  south  of  west.  In  the  first  case 
the  days  are  long  and  the  nights  short;  in  the  second  the 

days  are  short  and  the  nights  long. 
Any  one  who  thinks  will  see  that 
this  annual  change  of  the  seasons 
depends  in  some  way  on  the  sun; 
that  the  season  is  hot  or  cold,  and 
the  days  long  or  short,  according  to 
the  apparent  path  of  the  sun  in  the 
heavens.  We  have  now  to  show 
that  the  changes  of  the  seasons 
arise  from  the  earth  making  an 
annual  revolution  around  the  sun. 
Thus  the  earth  has  two  motions, 
its  rotation  on  its  own  axis,  and  its 
The  first  produces  day  and  night, 
the  second  summer  and  winter.  To  conceive  the  combined 
effect  of  these  two  revolutions  is  a  task  which  requires  some 
thinking.  We  have  two  things  to  consider,  —  the  actual  motion 


FIG.  16.  —  The  earth  moving 
round  the  sun. 

revolution  around  the  sun. 


REVOLUTION  OF  THE  EARTH  ROUND  THE  SUN  33 

of  the  earth,  and  the  apparent  motion  of  the  sun  as  we  seem 
to  see  it. 

If  we  could  fly  upward  in  a  direction  near  that  of  the 
earth's  axis  to  a  distance  of  a  thousand  million  miles,  and  then 
look  back,  we  should  see  the  earth  and  a  number  of  other 
bodies  forming,  as  it  were,  a  little  family  far  distant  from  all 
other  heavenly  bodies.  The  largest  and  brightest  of  those 
bodies  would  be  the  sun.  At  various  distances  and  directions 
from  the  sun  we  should  see  eight  or  more  smaller  bodies  look- 
ing like  stars.  If  we  watched  long  enough,  we  should  see  that 
those  seeming  stars  were  all  in  motion  around  the  sun,  each  one 
keeping  nearly,  but  not  exactly,  at  the  same  distance  from  it 
during  its  course.  The  nearest  would  complete  its  circuit  in 
about  three  months,  while  the  most  distant  would  take  more 
than  160  years.  These  small  starlike  bodies  are  called  planets. 

The  paths  in  which  the  planets  perform  their  courses  round 
the  sun  are  called  their  orbits. 

One  of  these  planets  is  the  earth  on  which  we  dwell.  It 
js_the  third  in  the  order  of  distance  from  the  sun,  and,  as  we 
have  said,  it  requires  a  year  to  complete  its  circuit  around  the 
sun.  It  would  be  more  exact  to  say  that  the  time  required 
for  it  to  complete  its  circuit  is  what  we  call  a  year. 

Besides  the  eight  planets  which  we  have  described  there 
are  a  number  of  smaller  bodies  going  round  the  sun  which  we 
shall  describe  hereafter.  This  whole  family  of  bodies  is 
called  the  solar  system.  It  is  so  called  because  the  sun  is  the 
great  central  body  on  which  all  the  others  depend,  and  to 
which  they  all  do  homage,  so  to  speak. 

The  distance  of  the  earth  from  the  sun  has  been  determined 
in  a  number  of  different  ways.  According  to  the  latest 
researches  it  is  very  nearly  93,000,000  miles ;  we  scarcely 
know  whether  a  little  greater  or  a  little  less.  Some  idea  of 
this  distance  may  be  gained  by  saying  that  a  railway  train 
running  GO  miles  an  hour,  and  making  no  stop,  would  require 
more  than  160  years  to  reach  the  sun.  Five  generations 
might  be  born  upon  it  before  the  journey  was  completed. 
NEWCOMB'S  ASTRON.  —  3 


34  ASTRONOMY 

The  most  marked  difference  between  the  sun  and  the  planets 
is  that  the  sun  shines  by  its  own  light,  while  the  planets  shine 
only  by  the  light  that  falls  on  them  from  the  sun.  Thus,  so 
far  as  means  of  seeing  are  concerned,  the  sun  is  like  a  candle 
in  an  otherwise  dark  room  and  the  planets  are  like  little 
bodies  seen  by  the  light  of  the  candle. 

We  have  said  that  the  bodies  of  the  solar  system  form  a 
group  by  themselves.  Looking  down  from  the  height  we 
have  supposed,  we  should  see  this  very  clearly.  The  stars 
which  stud  the  heavens  would  be  seen  just  as  we  see  them 
from  the  earth,  in  every  direction.  Their  distances  are  so 
vast  compared  with  the  size  of  the  solar  system  that  even  the 
latter,  immense  though  it  is,  is  but  a  speck  in  comparison.  We 
may,  if  we  please,  call  them  suns.  Most  of  them  are  brighter 
than  the  sun.  They  look  small  and  dim  because  they  are  so 
much  farther  away. 

Thus,  having  expanded  our  conceptions  so  that  the  earth 
shall  be  but  as  a  point  in  the  solar  system,  we  must  again  ex- 
pand them  so  as  to  think  of  the  whole  solar  system  as  but 
a  point  in  comparison  with  the  distance  of  the  stars. 

2.  Annual  Motion  of  the  Earth  round  the  Sun.  —  We  must 
now  explain  the  motion  of  the  earth  in  its  orbit  round  the  sun. 


This  is   called  its  annual  motion,  because  it  takes  a  year  to 
complete  one  revolution. 

We   cannot  draw  a  figure  which  shall  represent  the  earth 
and  its  orbit  in  anything  like  their  true  proportions,  because 


REVOLUTION   OF  THE  EARTH  ROUND   THE  SUN      35 

the  diameter  of  the  orbit  is  more  than  20,000  times  that 
of  the  earth  itself.  So  we  have  to  draw  figures,  as  before,  on 
two  very  different  scales.  Figure  16  shows  the  orbit  of  the 
earth  seen  nearly  edgewise.  The  plane  containing  this  orbit 
is  called  the  plane  of  the  ecliptic.  On  a  true  scale  the  earth  in 
this  figure  would  be  an  invisible  dot,  so  we  make  it  larger,  and 
then  represent  it  on  a  still  larger  scale  in  figure  17. 


Ait 


FIG.  17.  — Showing  how  the  sun  shines  on  the  earth  in  June,  illuminating 
the  whole  of  the  Arctic  circle,  while  the  whole  of  the  Antarctic  circle 
is  in  darkness. 

In  figure  16  the  direction  of  the  axis  is  shown  by  the 
inclined  line  NS. 

An  important  law  of  the  earth's  motion  is  this :  As  the 
earth  moves  round  the  sun,  the  direction  of  its  axis  remains  almost 
unchanged. 

This  direction  is  not  quite  perpendicular  to  the  ecliptic,  but 
is  inclined  to  the  perpendicular  by  23£°,  or  a  little  more  than 
one  fourth  of  a  right  angle.  This  angle  is  called  the  obliquity 
of  the  ecliptic,  because  it  is  equal  to  the  angle  which  the  plane 
of  the  equator  makes  with  the  plane  of  the  ecliptic. 


36  ASTRONOMY 

3.  How  the  Sun  shines  on  the  Earth  at  Different  Seasons.  — • 
Let  us  now  see  how  the  sun  shines  on  the  earth  at  different 
times  of  the  year. 

Spring  Position  of  the  Earth.  —  About  the  21st  of  March  of 
each  year  the  earth  is  in  the  position  A,  figure  16,  where  the 
line  from  the  sun  to  the  earth  is  at  right  angles  to  the  earth's 
axis.  The  sun  then  illuminates  the  whole  hemisphere  of  the 
earth  which  is  turned  toward  it,  from  pole  to  pole.  The  days 
and  nights  are  equal  all  over  the  earth.  This  time  is  called 
that  of  the  Vernal  Equinox,  because  the  season  in  our  hemi- 
sphere is  spring,  and  the  days  and  nights  are  equal. 

Summer  Position  of  the  Earth.  —  Three  months  later,  about 
June  21,  the  earth  will  be  in  the  position  B}  with  the  north 
end  of  its  axis  now  tipped  toward  the  sun.  The  sun  then 
shines  on  the  region  round  the  north  pole,  while  that  round  the 
south  pole  is  in  darkness,  as  we  see  by  figure  17,  which  repre- 
sents the  earth  in  the  position  B,  but  on  a  larger  scale. 

The  circle  AC  round  the  north  pole  of  the  earth,  which 
touches  the  edge  of  the  illuminated  hemisphere  at  this  time,  is 
called  the  Arctic  Circle.  Its  radius  will  be  23£°  of  the  earth's 
meridian,  the  same  as  the  obliquity  of  the  ecliptic.  As  the 
earth  revolves  on  its  axis  in  this  position,  the  region  within 
the  arctic  circle  will  never  be  carried  outside  of  where  the 
sun  is  shining.  Hence,  to  an  observer  in  this  region  the  sun 
will  not  set  on  the  21st  of  June,  but  will  seem  to  go  round  the 
sky  in  the  direction  from  south  through  west,  north,  and  east. 

Next,  imagine  a  circle,  MN,  drawn  round  the  south  pole  of 
the  earth,  so  as  to  touch  the  edge  of  the  illuminated  hemi- 
sphere. This  is  called  the  Antarctic  Circle.  We  see  that,  as 
the  earth  revolves,  the  region  within  this  circle  will  not  be 
brought  into  sunshine  at  all.  Hence  the  sun  will  never  rise 
within  this  circle  on  June  21. 

At  a  distance  of  23£°  north  of  the  equator  EQ,  there  is  a 
circle  FG  on  which  the  sun  will  be  in  the  zenith  at  noon  of 
June  21.  This  circle  is  called  the  Tropic  of  Cancer. 

At  the  equator  EQ  the  days  will  be  equal  to  the  nights. 


REVOLUTION  OF  THE  EARTH  ROUND  THE  SUN     37 

The  further  north  we  go  from  the  equator,  the  larger  the 
fraction  of  a  circle  of  latitude  round  the  earth  which  will  be 
in  sunshine.  Hence  on  the  21st  of  June  the  days  are  longer 
and  the  nights  shorter  as  we  go  toward  the  north. 

South  of  the  equator  the  days  get  shorter  and  the  nights 
longer,  as  we  travel  south,  until  we  reach  the  antarctic  circle, 
when  the  sun  will  simply  show  himself  on  the  horizon  at 
noon. 

Autumn  Position  of  the  Earth.  —  At  (7,  the  plane  of  the  equator 
again  passes  through  the  sun,  and  the  latter  shines  over  one 
hemisphere  of  the  earth,  from  the  north  to  the  south  pole.  At 
this  time  the  days  and  nights  are  again  equal  the  world  over. 
This  is  called  the  Autumnal  Equinox,  because  the  days  and 
nights  are  again  equal  and  the  season  is  autumn. 

Winter  Position  of  the  Earth.  —  On  December  21  the  earth 
is  in  the  position  D,  with  the  north  end  of  the  axis  tipped 
away  from  the  sun,  and  the  south  end  tipped  toward  it.  Now 
day  and  night  are  the  reverse  of  what  they  were  with  the 
earth  at  B.  Figure  17  will  still  answer  for  us,  only  it  is  now 
night  where  it  is  represented  as  day  in  the  figure,  and  vice 
versa.  All  the  region  within  the  arctic  circle  is  in  darkness, 
all  that  within  the  antarctic  circle  in  the  sunshine.  North  of 
the  equator  the  nights  are  longer  than  the  days;  south  of  it 
the  days  are  longer  than  the  nights. 

The  sun  passes  through  the  zenith  of  every  place  in  latitude 
23J°  south  at  noon  of  this  day.  This  circle  of  latitude  is  called 
the  Tropic  of  Capricorn. 

4.  Apparent  Motion  of  the  Sun.  —  The  Zodiac.  Having 
explained  how  the  earth  turns  on  its  axis  and  revolves  round 
the  sun,  while,  to  us  who  live  on  it,  it  seems  to  remain  at  rest, 
we  shall  now  explain  how  the  sun  seems  to  us  to  move.  The 
apparent  motion  of  the  sun  is  based  on  these  facts  :  — 

1.  Each  fixed  star  is  really  in  the  same  direction  from  us 
all  day  and  all  the  year.  The  stars  seem  to  us  to  change  their 
direction  only  because  we  live  on  the  moving  earth. 


38 


ASTRONOMY 


2.  The  sun  is  nearly,  but  not  exactly,  in  the  same  direction 
from  us  all  day,  from  its  rising  to  its  setting.  But  this  direc- 
tion changes  during  the  year  in  consequence  of  the  earth 
revolving  round  it. 


FIG.  18.  —  Showing  how,  in  consequence  of  the  earth  moving  around  the 
sun,  the  sun  seems  to  us  to  make  an  annual  revolution  round  the 
celestial  sphere  among  the  stars,  passing  through  the  twelve  signs  of 
the  zodiac. 

Let  us  study  figure  18,  which  shows  the  earth's  orbit,  ABC, 
with  the  sun  in  the  center.  Far  outside  the  orbit  lie  the  stars. 
To  make  a  figure  on  the  right  scale,  we  should  have  to  place 
the  stars  several  miles  away ;  as  we  cannot  do  this,  we  repre- 
sent their  positions  as  in  the  figure. 


REVOLUTION  OF  THE  EARTH  ROUND  THE  SUN     39 

Now  suppose  we  could  fly  a  few  thousand  miles  above  the 
earth  and  accompany  it  in  its  course  round  the  sun.  Then, 
looking  down,  we  should  see  the  earth  turning  on  its  axis,  and 
bringing  its  oceans  and  continents  into  view,  one  after  the 
other.  Looking  at  the  stars,  we  should  see  them  at  rest.  They 
would  neither  rise  nor  set,  nor  even  change  their  direction  by 
any  quantity  we  could  perceive. 

Next,  let  us  see  how  it  will  be  with  the  sun.  When  the 
earth  is  at  the  point  A,  we  shall  see  the  sun  as  if  it  were  among 
the  stars  at  the  point  a.  A  month  later  when  the  earth  has 
got  to  the  point  B,  the  sun  will  appear  among  the  stars  at  b. 
In  another  month,  with  the  earth  at  (7,  the  sun  will  be  seen  as 
if  at  c,  and  so  on  through  the  year.  As  the  earth  goes  through 
its  revolution  round  the  sun,  the  sun  appears  to  move  around 
in  a  circle  among  the  stars,  until  the  earth  gets  back  to  the 
position  A,  when  the  sun  will  again  appear  in  the  position  a. 
Hence :  — 

The  sun  appears  to  us  to  describe  a  complete  circle  around  the 
celestial  sphere,  among  the  stars,  every  year. 

The  circle  thus  described  by  the  sun  on  the  celestial  sphere 
is  called  the  ecliptic. 

The  zodiac  is  an  imaginary  belt  in  the  heavens,  extending  8° 
on  each  side  of  the  ecliptic,  and  passing  all  round  the  celestial 
sphere  as  the  ecliptic  does.  The  ecliptic  is  its  central  line. 

If  the  axis  of  the  earth  were  perpendicular  to  the  ecliptic, 
the  plane  of  the  earth's  equator  would  always  pass  through 
the  sun,  and  the  sun  would  always  be  seen  in  the  celestial 
equator.  Because  of  the  obliquity  of  the  ecliptic,  already 
described,  the  ecliptic  is  inclined  to  the  equator  by  an  angle  of 
23£°,  cutting  it  at  two  points  called  the  Vernal  and  Autumnal 
equinoxes,  as  shown  in  figure  19. 

To  make  this  clear,  we  show  in  figure  20  how,  if  we  could 
see  the  stars  around  the  sun,  and  the  ecliptic  and  equator 
marked  on  the  celestial  sphere,  we  should,  day  by  day,  see  the 
sun  moving  from  west  toward  east,  among  the  stars. 


40 


ASTRONOMY 


In  very  ancient  times  men  mapped  out  the  apparent,  course 
of  the  sun  round  the  celestial  sphere,  as  shown  in  figure  19. 
They  divided  it  into  twelve  parts,  each  30°  in  length,  and 


FIG.  19.  —  Showing  how  the  celestial  equator  and  the  ecliptic  span  the 
celestial  sphere  among  the  stars,  the  two  being  inclined  at  an  angle 
of  23*°. 

Not  only  the  earth,  but  the  whole  solar  system,  must  be  conceived 
as  a  point  in  the  center  of  the  figure.  We  must  imagine  ourselves 
looking  out  from  this  center.  Then  if  we  could  see  the  stars  around 
the  sun  we  should  see  the  latter  appearing  to  pass  around  the  ecliptic 
through  the  signs  of  the  zodiac,  as  marked  in  the  figure. 


March  22 


Ma  re h  2/ 


Equcrft 


March  20 


March  /9 


FIG.  20.  -  The  sun  crossing  the  equator  about  March  20. 

named  each  part  after  the  constellation   in  which  the  sun 
would  have  been  seen  had  the  stars  been  visible.     These  parts 


REVOLUTION  OF  THE  EARTH  ROUND   THE  SUN     41 

were  called  signs  of  the  zodiac.  The  sun  enters  a  sign  about 
the  21st  day  of  each  month.  The  names  of  the  signs  and  the 
months  when  the  sun  enters  each  are  as  follows :  — 

Aries,  The  Ram         .        ,        ,        .        .  March 

Taurus,  The  Bull      .        .        .        .        .  April 

Gemini,  The  Twins May 

Cancer,  The  Crab      •        •  June 

Leo,  The  Lion  ......  July 

Virgo,  The  Virgin      .        .        .  • .  August 

Libra,  The  Balance September 

Scorpio,  The  Scorpion        .        .        .        .  ,  October 

Sagittarius,  The  Archer  ;        •        .  November 

Capricorn  us,  The  Goat      .        .        .  December 

Aquarius,  The  Water  Bearer    .        .        .  January 

Pisces,  The  Fishes     .        .        .        .        .  February 

When  the  sun  is  at  the  Vernal  Equinox,  it  appears  in  the 
celestial  equator,  rises  exactly  east,  and  sets  exactly  west. 

In  figure  19  we  see  that  during  the  six  months  the  sun  is 
passing  from  Aries  to  Virgo,  it  appears  north  of  the  celestial 
equator.  It  is  therefore  in  north  declination ;  it  rises  north 
of  east  and  sets  north  of  west.  At  this  time,  in  the  northern 
hemisphere,  the  days  are  longer  than  the  nights.  See  the 
apparent  diurnal  course  of  the  sun  as  shown  in  figure  22. 

When  the  sun  passes  from  Gemini  into  Cancer,  it  has 
reached  its  greatest  north  declination,  and  now  begins  to  move 
south  again.  This  point  is  called  the  Summer  Solstice. 

When  the  sun  reaches  Libra,  it  again  crosses  the  equator 
toward  the  south.  This  point  is  called  the  Autumnal  Equinox. 

During  the  remaining  six  months,  while  the  sun  is  passing 
from  Libra  to  Pisces,  it  is  in  south  declination ;  it  rises  south  of 
east  and  sets  south  of  west.  In  the  northern  hemisphere  the 
nights  are  then  longer  than  the  days. 

When  the  sun  passes  from  Sagittarius  into  Capricornus 
it  has  reached  its  greatest  south  declination,  and  begins  to 
return  toward  the  equator.  This  point  is  called  the  Winter 
Solstice. 


42 


ASTIiONOMY 


5.  Seasons  in  the  Two  Hemispheres.  —  The  reason  that  sum- 
mer is  hotter  than  winter  is  that  the  sun  when  north  of  the 
equator,  not  only  shines  longer  upon  us  every  day,  but  is 
nearer  the  zenith  at  noon.  Thus  more  of  its  heat  falls  on 
any  given  surface  —  a  square  mile,  for  example,  as  shown  in 
figure  21. 

As  the  sun  moves  south  in  declination,  its  rays  fall  upon  our 
portion  of  the  earth  at  a  greater  obliquity,  so  that  every  square 
mile  of  our  country  receives  less  heat  day  by  day. 


FIG.  21.  —  Showing  how  a  square  mile  of  the  earth  receives  less  heat,  the 
nearer  the  sun  is  to  the  horizon.  When  the  sun  is  in  the  zenith,  the 
region  BC  receives  as  many  of  his  rays  as  the  region  .4(7,  twice  as 
large,  receives  when  the  altitude  of  the  sun  is  30°. 

The  greatest  amount  of  heat  is  received  at  the  time  of  the 
summer  solstice,  about  June  21,  and  the  least  at  the  winter 
solstice,  December  22.  But  the  highest  average  temperature 
does  not  occur  till  July.  This  is  because  the  sun's  rays  re- 
quire time  in  order  to  warm  up  the  air  and  the  surface  of  the 
land  and  sea,  much  as  it  takes  time  for  a  fire  to  warm  up  a 
room.  The  lowest  temperature  does  not  occur  till  January, 
because  earth,  air,  and  ocean  retain  for  some  time  the  heat 
radiated  to  them  during  the  preceding  months. 


REVOLUTION   OF  THE  EARTH  ROUND   THE  SUN     43 


But  in  the  southern  hemisphere  the  seasons  are  reversed. 
When  the  sun  is  in  south  declination,  as  at  the  winter  solstice, 
the  southern  hemisphere  has  the  longest  days  and  the  shortest 
nights.  Hence,  during  our  winter  in  the  northern  hemisphere, 
the  southern  hemisphere  has  its  summer,  and  it  has  its  winter 
during  our  summer. 


South 


ASorffi 


FIG.  22.  —  Showing  the  apparent  diurnal  course  of  the  sun,  as  we  see  it  in 
our  latitudes  at  different  times  of  the  year.  You  must  fancy  yourself 
standing  in  the  center  of  the  landscape.  Then  in  summer  you  will 
see  the  sun  rise  considerably  north  of  east,  pass  not  far  south  of 
the  zenith  at  noon,  and  set  to  the  north  of  west,  as  shown  in  the 
right  hand  circle  of  the  figure.  During  the  night  it  is  completing  that 
part  of  the  circle  which  is  below  the  horizon.  During  the  remaining 
months  of  the  year  it  seems  to  pass,  day  by  day,  farther  and  farther 
toward  the  south  until  December,  when  it  seems  to  describe  the  left 
hand  circle.  It  then  rises  south  of  east  and  sets  south  of  west.  We 
see  that  at  this  time  the  greater  part  of  the  circle  is  below  the  horizon, 
while  in  June  the  greater  part  is  above  the  horizon. 


44  ASTRONOMY 

We  know  that  on  a  general  average  the  hottest  climates  are 
within  the  tropics,  and  that  the  temperature  is  lower  toward 
either  pole.  This  is  because  the  obliquity  of  the  sun's  rays 
increases  toward  the  poles.  At  the  poles  the  sun  shines  only 
half  the  year,  and  then  is  never  more  than  23£°  above  the 
horizon. 

6.  The  Solar  and  Sidereal  Years. — There  are  two  ways  of 
finding  how  long  it  takes  the  sun  to  complete  its  apparent  rev- 
olution in  the  heavens,  or,  in  other  words,  how  long  it  takes 
the  earth  to  make  a  complete  revolution  round  it. 

One  of  these  consists  in  observing  the  exact  time  at  which 
the  sun  reaches  the  equinoxes.  In  ancient  times  astronomical 
observers  were  able  to  do  this  by  noting  the  days  when  the  sun 
rose  exactly  in  the  east  or  set  exactly  in  the  west.  By  observ- 
ing the  rising  and  setting  from  day  to  day,  they  could  find  not 
only  the  day,  but  almost  the  hour  in  which  the  sun  was  on  the 
celestial  equator.  Of  course,  with  our  more  exact  instruments, 
we  can  get  this  time  with  still  greater  precision. 

The  period  between  two  returns  of  the  sun  to  the  same  equi- 
nox is  called  the  solar  year  or  equinoxial  year. 

The  other  way  of  finding  the  length  of  the  year  consists  in 
observing  the  interval  of  time  between  two  passages  of  the  sun 
past  the  same  star  in  the  heavens;  for  example,  the  period 
between  two  of  its  passages  past  one  of  the  stars  shown  in 
figure  20. 

This  method  seems  to  involve  the  great  difficulty  that  we 
cannot  see  when  the  sun  is  near  the  star.  But  the  astronomer 
has  methods  of  knowing  exactly  where  a  star  is  by  day  as  well 
as  by  night,  and  can  determine  the  moment  at  which  the  sun 
passes  it. 

The  ancient  astronomers  got  the  same  result  by  using  the 
moon  as  an  intermediate  object  to  measure  from.  The  moon 
could  be  seen  before  sunset  and  its  distance  from  the  sun  de- 
termined. Then,  when  the  star  appeared  after  sunset,  the 
distance  from  the  star  to  the  moon  was  measured.  Allowing 


REVOLUTION  OF  THE  EARTH  ROUND  THE  SUN     45 

for  the  motion  of  the  moon  during  the  interval,  the  apparent 
distance  between  the  sun  and  the  star  could  thus  be  learned 
from  day  to  day.  In  this  way  it  could  be  found  how  many 
days  it  was  between  the  times  at  which  the  sun  was  at  the 
same  distance  from  any  given  bright  star.  This  would  be  the 
period  of  apparent  revolution  of  the  sun  in  the  celestial 
sphere,  or,  as  we  now  know  it  to  be,  the  period  of  one  revolu- 
tion of  the  earth  in  its  orbit.  This  period  is  called  the  sidereal 
year,  because  it  is  fixed  by  the  stars. 

Hipparchus,  who  flourished  about  150  B.C.,  was  the  first  to 
make  exact  observations  of  the  length  of  the  year.  Ptolemy, 
who  flourished  about  300  years  later,  made  similar  ones.  They 
found  that  the  length  of  the  year,  as  determined  in  these  two 
ways,  was  not  the  same,  and  that  the  solar  year,  as  determined 
by  the  equinoxes,  was  several  minutes  shorter  than  the  side- 
real year  determined  by  the  return  of  the  sun  to  the  same 
star. 

With  our  exact  modern  observations  we  have  found  the 
lengths  of  the  years  to  be :  — 

Solar  year,         365  d.  5  h.  48  m.  46  s. 
Sidereal  year,   365       699 

Difference,  20m.  23s. 

This  difference  shows  that  the  position  of  the  equinoxes 
among  the  stars  is  changing  from  year  to  year.  Hipparchus 
and  Ptolemy  estimated  the  change  to  be  about  one  degree  in 
a  century.  We  know  it  to  be  greater  than  this, — nearly  one 
degree  in  70  years. 

7.  Precession  of  the  Equinoxes.  —  The  motion  of  the  equi- 
noxes which  causes  the  difference  between  the  solar  and  side- 
real year  is  going  on  all  the  time.  It  is  called  the  Precession 
of  the  Equinoxes. 

The  nature  of  precession  is  now  to  be  explained.  The  equi- 
nox is  the  point  where  the  sun  crosses  the  celestial  equator. 
The  position  of  the  celestial  equator  on  the  celestial  sphere 


46  ASTRONOMY 

is  determined  by  the  direction  of  the  earth's  axis,  because  the 
celestial  equator  is  90°  from  either  celestial  pole. 

The  precession  of  the  equinoxes  arises  from  the  fact  that  the 
direction  of  the  earth's  axis  in  space  is  slowly  changing. 

Next,  let  us  see  how  the  change  goes  on.  Imagine  a  line 
passing  through  the  sun  perpendicular  to  the  plane  of  the 
ecliptic.  The  point  in  which  this  line,  when  continued  to  the 
stars,  meets  the  celestial  sphere,  is  called  the  Pole  of  the  Edip- 
tic.  It  lies  in  the  constellation  Draco,  the  Dragon,  but  there 
is  no  bright  star  near  it. 


2000  years  ago 


Ce/e-sr/a/  Equator  A.D.  /9OO 


FIG.  23.  —  Showing  how  the  equinoxes  are  gradually  shifting  in  conse- 
quence of  the  motion  of  the  celestial  equator  among  the  stars.  One 
of  the  brightest  stars  in  the  figure,  which  was  south  of  the  equator 
two  thousand  years  ago,  is  now  north  of  it. 

You  will  readily  see  that  the  angular  distance  between  the 
pole  of  the  ecliptic  and  the  celestial  pole,  corresponding  to 
the  direction  of  the  earth's  axis,  is  equal  to  the  obliquity  of 
the  ecliptic,  23J°. 

Now,  the  law  of  precession  is  that  the  celestial  pole  is  in 
motion,  and  makes  a  complete  revolution  round  the  pole  of 
the  ecliptic  in  about  25,700  years.  This  motion  is  very  slow 
to  ordinary  vision ;  it  would  take  a  century  for  the  naked  eye 
to  notice  it,  even  by  careful  observation.  But  the  exact  obser- 


REVOLUTION  OF  THE  EARTH  ROUND  THE  SUN  47 

vations  made  by  astronomers  with  the  meridian  circle  make  it 
evident  month  after  month  and  year  after  year. 

Owing  to  this  motion  of  the  celestial  pole  the  celestial 
equator  moves  also,  continually  sliding  along  the  ecliptic,  and 
carrying  the  equinoxes  with  it,  as  shown  in  figure  23.  This  is 
why  the  equinox  moves  among  the  stars.  The  rate  of  motion 
is  a  little  more  than  50"  in  a  year,  or  nearly  14°  in  1000  years. 

Motion  of  the  Ecliptic.  —  If  the  plane  of  the  ecliptic  were 
absolutely  fixed,  the  obliquity  of  the  ecliptic  would  be  always 
the  same,  and  the  motion  of  precession  would  go  on  forever 
at  the  same  rate  that  it  now  does.  But  the  attraction  of  the 
other  planets  on  the  earth  produces  a  very  slow  change  in  the 
ecliptic  itself,  about  -5^  the  change  of  precession.  In  conse- 
quence of  this  change,  the  revolution  of  the  celestial  pole 
round  the  pole  of  the  ecliptic  does  not  take  place  at  an 
exactly  uniform  rate,  nor  will  it  always  be  completed  in 
3xactly  the  same  time.  For  the  same  reason  the  obliquity  of 
the  ecliptic  slowly  changes.  It  is  at  the  present  time  dimin- 
ishing at  the  rate  of  about  46"  in  a  century. 

Results  of  Precession.  —  One  result  of  precession  is  that  the 
celestial  pole  was  not  so  near  the  polestar  in  former  times  as 
it  is  now.  In  ancient  times  it  was  so  far  away  from  that  star 
that  the  latter  could  not  be  considered  as  a  polestar  at  all.  It 
has  been  continually  coming  nearer,  and  is  still  approaching 
it.  About  the  year  2110  it  will  pass  by  the  polestar  at  a  dis- 
tance of  only  24'.  Continuing  its  course,  the  celestial  pole 
will  pass  some  5°  from  the  star  Alpha  Lyrse,  about  11,000  years 
from  now,  and  will  continue  its  circuit  until  it  gets  back  to 
where  it  now  is  in  about  25,700  years. 

The  two  equinoxes  will  make  a  revolution  round  the  equator 
in  the  same  period  of  time,  being  carried  along  by  the  earth?s 
equator,  which  is  always  at  right  angles  to  the  earth's  axis. 


CHAPTER   III 


OF  TIME 

1.  Diurnal  Motion  of  the  Sun  and  Stars.  —  We  now  know 
why  it  is  that  we  do  not  see  the  same  stars  every  evening  all 
the  year  round.  A  star  which,  at  any  time,  is  seen  in  the  west 
after  sunset,  will,  evening  after  evening,  be  seen  nearer  and 
nearer  the  sun,  until  it  is  lost  in  the  sun's  rays.  Then,  when 
the  sun  has  got  considerably  past  it,  we  shall  see  it  in  the 
morning  before  sunrise. 


FIG.  24. 

Imagine  ourselves  seeing  the  sun  pass  the  meridian  to-day. 
Suppose  any  star  above  it  passing  the  meridian  at  the  same 
moment.  To-morrow,  at  noon,  the  sun  will  have  moved  a  little 
east  of  the  star  (figure  24).  Hence  the  star  will  pass  the 
meridian  before  the  sun  does.  Next  day  it  will  pass  earlier 
than  the  sun  by  a  yet  greater  amount,  and  so  on  through  the 
entire  year.  At  the  end  of  the  year  they  will  again  pass  the 
meridian  together.  You  see  from  this  that  the  star,  in  its 

48 


OF  TIME  49 

apparent  diurnal  revolution,  has  been  continually  running 
ahead  of  the  sun  and  has  caught  up  to  it  from  behind  at  the 
end  of  the  year.  It  follows  that,  in  the  course  of  the  year,  the 
star  will  have  risen,  crossed  the  meridian,  and  set  one  time  more 
than  the  sun.  The  sun  makes  365J  apparent  diurnal  revolu- 
tions around  the  earth,  there  being  one  revolution  a  day.  It 
follows  that  the  star  will  have  made  366J  apparent  diurnal 
revolutions. 

If  we  divide  the  number  of  seconds  in  a  day  by  365},  it  will 
give  us  the  time  by  which  the  star  has  gained  on  the  sun  every 
day.  We  find  the  quotient  to  be  237  seconds,  or  3  m.  57  s. 
Subtracting  this  from  24  hours,  we  find  the  apparent  diurnal 
revolution  of  the  stars  to  be  made  in  23  h.  56  m.  3  s.,  or  nearly 
4  minutes  less  than  a  day.  Hence,  any  star  passes  the  meridian 
three  minutes  and  fifty-seven  seconds,  or  nearly  four  minutes, 
earlier  every  day  than  it  did  the  day  before. 

The  time  between  two  successive  passages  of  a  star  over  the 
meridian  is  called  a  sidereal  day,  which  means  star  day. 

Astronomers  divide  the  sidereal  day  into  24  sidereal  hours, 
each  hour  into  60  sidereal  minutes,  and  so  on. 

It  will  be  seen  from  the  way  we  have  described  the  equi- 
noxes that  they  each  mark  a  certain  point  among  the  stars 
on  the  celestial  sphere,  and  therefore  that  each  equinox,  like 
a  star,  crosses  the  meridian  every  day  about  3  m.  57  s.  earlier 
than  it  did  the  day  before.  When  the  vernal  equinox  crosses 
the  meridian  of  a  place,  it  is  called  sidereal  noon  at  that  place. 

Sidereal  time  is  the  number  of  sidereal  hours,  minutes,  and 
seconds  since  the  vernal  equinox  crossed  the  meridian.  It  is 
counted  from  0  h.  to  24  h. 

A  sidereal  clock  is  a  clock  so  regulated  as  to  keep  sidereal 
time.  Its  pendulum  is  a  little  shorter  than  that  of  a  common 
clock,  so  that  it  shall  gain  3  m.  57  s.  a  day  on  the  latter.  The 
hands  being  set  so  that  they  shall  read  0  h.  0  m.  0  s.  as  the  ver- 
nal equinox  crosses  the  meridian,  the  successive  passages  of 
the  24  principal  hour  circles  over  the  meridian  are  told  off  by 
the  clock.  By  looking  at  the  clock,  the  astronomer  can  deter- 
NEWCOMB'S  ASTRON. — 4 


60 


ASTRONOMY 


mine  at  any  moment  the  position  of  any  constellation  relative 
to  his  meridian,  and  can  point  his  telescope  at  any  star  he  wants 
to  see  by  day  as  well  as  by  night. 

2.  Mean  and  Apparent  Time ;  Inequality  of  Apparent  Time.  — 
The  measure  of  time  which  we  use  in  daily  life  is  called  civil 
time.  The  moment  when  the  sun  crosses  our  meridian  we  call 
noon.  But  there  is  a  difficulty  in  using  the  true  noon  as  12 
o'clock,  owing  to  the  obliquity  of  the  ecliptic  and  the  unequal 
motion  of  the  earth  round  the  sun. 


FIG.  25.  —  Showing  the  reason  of  the  equation  of  time. 

The  earth  moves  a  little  faster  in  its  orbit  in  our  winter  than 
it  does  in  our  summer.  Hence  the  sun  seems  to  move  along  in 
the  ecliptic  a  little  faster  in  winter  than  in  summer.  Owing 
to  the  obliquity  of  the  ecliptic  the  earth  sometimes  has  to  turn 
farther  in  order  that  a  meridian  may  catch  up  to  the  sun,  than 
it  does  at  other  times. 

Figure  25  shows  this.  Suppose  that  on  some  day  at  noon  near 
the  vernal  equinox  we  see  the  sun  at  R.  Next  day  the  point 
R  being  fixed  among  the  stars  will  pass  the  meridian  3  m.  57  s. 
before  noon,  and  the  sun  will  be  at  S,  having  moved  obliquely 
toward  the  north.  In  order  that  the  sun  S  may  reach  the 
meridian  TR9  it  will  have  to  pass  over  the  distance  ST  by  its 


OF  TIME  51 

apparent  diurnal  motion ;  or,  in  other  words,  the  meridian  TR 
will  have  to  pass  over  the  distance  TS  to  be  at  the  sun.  But 
the  line  ST  is  shorter  than  SR.  Hence  it  will  take  the  sun 
less  than  3  m.  57  s.  to  pass  from  S  to  T,  so  that  it  will  be  on 
the  meridian  a  little  earlier  than  it  was  the  day  before. 

Next  suppose  the  sun  near  the  summer  solstice.  On  account 
of  the  convergence  of  the  meridians  from  the  equator  toward 
the  north  pole,  the  sun  will  pass  over  more  than  3  m.  57  s.  of 
right  ascension  near  the  solstices,  and  so  will  pass  the  meridian 
later  each  day  than  the  day  before. 

In  consequence  of  these  two  inequalities,  the  sun,  at  certain 
times  of  the  year,  falls  behind,  little  by  little,  day  after  day, 
and  at  other  times  it  catches  up  again,  making  the  times 
between  noons  longer  at  some  seasons  than  at  others.  Hence, 
if  we  used  time  measured  by  the  true  position  of  the  sun,  our 
hours  would  be  of  slightly  unequal  length.  This  unequal  time, 
measured  by  the  true  sun,  is  called  apparent  time.  The  moment 
when  the  real  sun  is  on  the  meridian  is  called  apparent  noon. 

In  former  times,  when  people  did  not  have  good  watches  or 
clocks,  and  the  exact  time  was  not  important  to  know,  they 
generally  went  by  the  sun  in  setting  their  timepieces.  But 
owing  to  the  inequality  of  the  intervals  between  two  apparent 
noons,  a  timepiece  will  not  keep  apparent  time. 

Mean  Time.  —  To  make  the  hours  of  equal  length,  we  fancy 
an  imaginary  sun  to  move  round  the  celestial  equator  at  a 
uniform  rate,  so  that  the  true  sun  shall  be  sometimes  ahead 
of  and  sometimes  behind  the  imaginary  sun.  The  latter  is 
called  the  mean  sun. 

When  the  mean  sun  passes  our  meridian,  it  is  called  mean 
noon.  Time  measured  from  mean  noon  to  mean  noon  is  called 
mean  time.  This  is  the  only  kind  of  time  we  can  measure  with 
a  clock,  and  it  is  the  only  kind  now  in  general  use. 

Equation  of  Time.  —  The  difference  between  apparent  time 
and  mean  time  is  called  the  equation  of  time.  It  is  greatest 
early  in  November  of  every  year,  when  the  true  sun  crosses  the 
meridian  about  16  minutes  before  mean  noon.  In  February, 


52  ASTRONOMY 

the  true  sun  is  nearly  as  far  behind  the  mean  sun  and  crosses 
the  meridian  about  14  minutes  after  mean  noon.  Thus  the 
greatest  mistake  we  should  make  in  measuring  time  by  the  true 
sun  would  be  about  a  quarter  of  an  hour. 

Some  almanacs  give  the  equation  of  time  for  every  day  in 
the  year,  or,  which  amounts  to  the  same  thing,  the  time  to  which 
you  should  set  your  clock  every  day  at  the  moment  when  the 
sun  is  on  the  meridian.  The  following  examples  will  make 
this  clear  :  — % 

February  11,  sun  on  meridian  at  12  h.  14  m.  mean  time 

April  15,  sun  on  meridian  at  12  0  mean  time 

May  14,  sun  on  meridian  at  11  56  mean  time 

June  14,  sun  on  meridian  at  12  0  mean  time 

July  26,  sun  on  meridian  at  12        6  mean  time 

September  1,  sun  on  meridian  at  12  0  mean  time 

November  3,  sun  on  meridian  at  11  44  mean  time 

December  25,  sun  on  meridian  at  12        0  mean  time 

We  see  that  there  are  four  days  in  the  year  when  the  sun  is 
on  the  meridian  at  mean  noon,  so  that  the  mean  and  apparent 
time  are  then  the  same. 

3.  Local  Time  and  Longitude.  —  As  the  earth  revolves  on  its 
axis,  all  its  meridians  in  succession  pass  the  sun,  or,  as  it 
appears  to  men,  the  sun  passes  all  the  meridians  in  its  apparent 
diurnal  motion  round  the  earth.  Because  it  is  noon  when  the 
sun  is  on  the  meridian  of  a  place,  we  see  that  noon  is  contin- 
ually traveling  round  the  earth,  getting  back  to  the  same  place 
in  24  hours.  The  circumference  of  the  earth  being  360°,  we 
find,  by  division,  that  noon  travels  round  the  earth  at  the  rate 

of 

15°  in  1  hour  of  time 
15'  in  1  minute  of  time 
15"  in  1  second  of  time 

In  the  latitude  of  the  middle  states,  1'  of  longitude  is  about 
4800  feet.  Hence,  15"  is  about  1200  feet.  Thus  we  see  that, 
in  our  latitude,  noon  travels  from  east  to  west  at  the  rate  of 


OF  TIM B  SB 

about  1200  feet  a  second.  It  requires  between  4  and  5  seconds 
to  travel  a  mile.  Hence,  two  places  do  not  have  the  same  time 
at  the  same  real  moment  unless  they  are  north  and  south  of 
each  other. 

Time  measured  at  any  place  from  the  noon  of  that  place  is 
called  local  time.  Hence  the  local  time  of  two  places  a  mile 
east  and  west  of  each  other  will  differ  in  our  latitude  between 
4  and  5  seconds.  As  there  are  3600  seconds  to  an  hour,  it  fol- 
lows that  at  two  places  800  miles  east  and  west  of  each  other, 
the  difference  of  time  will  be  about  an  hour. 

4.  Standard  Time.  —  When  people  traveled  by  stage  coaches 
or  sailing  vessels,  and  did  not  have  very  good  watches,  this 
difference  of  local  time  in  different  places  caused  them  no 
trouble.  When  a  man  drove  by  stage  to  another  place  he  set  his 
watch  on  the  new  time.  But  when  railroads  got  into  opera- 
tion, so  that  a  man  could,  in  the  course  of  a  day,  travel  from 
one  place  to  another  where  the  time  was  half  an  hour  or  more 
different,  it  was  very  troublesome.  Nearly  every  railroad 
chose  the  time  of  one  of  the  principal  cities  through  which  it 
passed,  and  thus  it  happened  that  travelers  would  frequently 
miss  a  train  by  mistaking  the  time. 

To  remedy  this  inconvenience,  our  system  of  standard  time 
was  introduced  in  1883.  Four  standard  meridians  through  the 
United  States  were  chosen,  75°,  90°,  105°,  and  120°  west  of 
Greenwich.  By  looking  at  a  map  of  the  United  States  we 
may  see  where  these  meridians  run. 

The  eastern  meridian,  75°  west  of  Greenwich,  passes  through 
central  New  York,  and  a  little  east  of  Philadelphia,  between 
that  city  and  Trenton. 

The  central  meridian,  90°  west,  passes  through  Wisconsin 
a  little  west  of  Madison,  and  down  the  Mississippi  Valley 
through  New  Orleans. 

The  meridian  of  105°  passes  along  the  eastern  slope  of 
the  Rocky  Mountains,  through  Wyoming,  Colorado,  and  New 
Mexico. 


54  ASTRONOMY 

The  meridian  of  120°  passes  through  Washington,  Oregon, 
and  California,  entering  the  Pacific  Ocean  on  the  coast  of  the 
latter  state. 

The  local  time  of  any  one  of  these  meridians  is  called  standard 
time.  To  see  how  it  compares  with  local  time,  as  already  de- 
fined, suppose  a  telegraphic  operator  anywhere  on  the  75th 
meridian  to  signal  the  exact  moment  at  which  mean  noon 
reaches  his  meridian  to  all  the  people  east  of  longitude  82£°. 
When  these  people  hear  his  signal  they  set  their  watches  at  12 
o'clock,  so  that  they  will  all  agree.  The  time  their  watches 
then  keep  is  called  Eastern  Time. 

One  hour  later,  mean  noon  has  reached  the  90th  meridian. 
At  this  moment  we  fancy  a  telegraph  operator  to  send  a  signal 
to  every  one  within  7£°  of  that  meridian,  that  is  every  one 
living  between  82^°  and  97£°  of  longitude.  These  people  all 
set  their  watches  at  12  the  moment  they  hear  his  signal.  At 
this  moment  it  will  be  1  o'clock  by  all  the  eastern  watches,  so 
that  the  watches  in  question  will  be  one  hour  behind  the  east- 
ern ones.  The  time  they  keep  is  called  Central  Time. 

The  people  in  the  Rocky  Mountain  region,  including  all  those 
between  97£°  and  112£°  of  longitude,  wait  another  hour,  till 
mean  noon  has  reached  the  meridian  of  105°,  and  then  all,  at 
the  same  moment,  set  their  watches  at  12.  The  time  they 
keep  is  called  Mountain  Time. 

In  another  hour  mean  noon  has  reached  the  meridian  120° 
west.  At  this  moment  all  the  people  in  the  region  west  of 
112£°,  which  includes  the  whole  Pacific  Coast,  set  their  watches 
at  12.  The  time  they  keep  is  called  Pacific  Time. 

We  see  that 

at  12  o'clock  Pacific  time 
it  is  1  o'clock  Mountain  time 
and  2  o'clock  Central  time 
and  3  o'clock  Eastern  time. 

Hence  the  standard  times  differ  only  by  one,  two,  or  three 
hours.     When  one  travels  from  San  Francisco  to  New  York,  if 


OF  TIME  55 

he  wants  his  watch  to  keep  the  time  of  each  region  through 
which  he  passes,  he  sets  it  an  hour  forward  as  he  enters  each 
region.  If  he  travels  west,  he  must  put  it  back  as  he  enters 
each  region.  These  jumps  of  one  hour  in  standard  time  are 
arranged  for  our  convenience,  so  that  when  we  travel  we  shall 
not  have  to  use  a  different  time  at  every  town. 

Understand  clearly  the  difference  between  local  and  standard 
time.  The  former  changes  constantly  as  we  travel  east  or 
west,  because  our  meridian  is  then  constantly  changing.  Sun- 
rise and  sunset  are  given  in  local  time,  because  they  constantly 
travel  from  east  to  west  around  the  earth.  Hence,  if  a  watch 
is  set  by  the  time  of  sunrise  or  sunset  as  we  find  it  in  an 
almanac,  it  will  not  give  standard  time  unless  we  are  on  one 
of  the  standard  meridians. 

The  difference  between  standard  and  local  time  is  greatest 
at  places  half  way  between  two  standard  meridians.  Here  the 
people  can  choose  which  meridian  they  will.  If  they  choose 
the  meridian  next  east  of  them,  their  watches  will  be  half  an 
hour  fast  of  local  time ;  if  they  choose  that  next  west,  they 
will  be  half  an  hour  slow.  Cincinnati,  for  example,  is  in  lon- 
gitude 841°.  This  is  5£°  east  of  the  central  meridian,  corre- 
sponding to  22  minutes  of  time.  Hence,  central  time  is  there 
22  minutes  behind  local  time,  so  that  noon  at  Cincinnati  occurs 
22  minutes  before  12  o'clock  central  time 


CHAPTER  IV 
OBSERVATION  AND  MEASUREMENT  OF  THE   HEAVENS 

1.  Refraction  of  Light.  —  When  rays  of  light  enter  a  trans- 
parent substance,  as  water  or  glass,  in  an  oblique  direction, 
they  are  bent  toward  the  direction  of  the  perpendicular,  as 
shown  in  figure  26.  When  they  pass  out  of  such  a  substance, 
they  are  bent  away  from  the  perpendicular,  as  shown  at  the 
bottom  of  the  figure.  This  bending  of  light  on  entering  or 
leaving  a  transparent  medium  is  called  refraction. 


FIG.  26.  —  Showing  the  refraction  of  light  in  passing  through  glass,  or  any 
other  transparent  substance.  When  it  enters  the  glass  the  rays  are 
bent  toward  the  perpendicular  ;  when  it  leaves  it  they  are  bent  from 
the  perpendicular. 

A  familiar  effect  of  refraction  is  seen  in  the  bent  appearance 
of  an  oar  or  a  straight  stick  when  held  obliquely  with  its  end 
under  water.  A  clear  pool  looks  shallower  than  it  really  is,  for 
the  same  reason. 

Atmospheric  Refraction.  —  Kef raction  may  be  produced  by  a 
gas  as  well  as  by  a  solid  body.  If  the  gas  is  unequally  dense 
in  its  various  portions,  a  ray  passing  through  it  is  refracted 
toward  the  denser  portion.  Now  the  air  is  more  and  more  dense 


OBSERVATION  AND  MEASUREMENT 


57 


FIG.  27.  —  Showing  why  a  pool  of  water  looks  shallower  than  it  really  is 
in  consequence  of  refraction.  The  looker-on  imagines  the  bottom  to 
be  in  a  straight  line  in  which  he  is  looking,  while  in  reality  it  is  in  the 
direction  of  a  bent  line  and  lower  down. 

from  its  upper  limit  to  the  surface  of  the  earth.  Hence,  when 
a  ray  of  light  passes  through  it,  it  is  bent  by  refraction,  so  that 
its  course  is  concave  toward  the  surface  of  the  earth.  This  is 
shown  in  figure  28,  which  represents  the  earth  surrounded  by 


FIG.  28.  —  Showing  how  the  sun's  light  is  refracted  by  the  atmosphere  so 
as  to  illuminate  the  region  within  the  earth's  shadow.  If  the  rays  of 
the  sun  went  in  straight  lines  without  refraction,  they  would  pass 
through  the  point  J5,  but,  in  consequence  of  refraction,  they  are  bent 
down  in  the  direction  G.  Hence,  they  meet  each  other  before  they 
reach  the  moon,  and  at  the  distance  of  the  moon  the  whole  interior 
of  the  shadow  is  illuminated  with  a  lurid  light. 


58  ASTRONOMY 

its  atmosphere.  As  a  ray  passes  through  the  air,  instead  of 
continuing  in  the  straight  line  AB,  it  is  gradually  bent  so  as 
to  leave  the  atmosphere  in  the  direction  AC.  This  bending  is 
called  atmospheric  refraction.  When  the  light  comes  from  a 
heavenly  body,  this  refraction  by  the  air  is  called  astronomical 
refraction. 

We  always  see  a  body  in  the  direction  from  which  the  light 
it  emits  or  reflects  reaches  our  eyes.  Thus,  as  a  result  of 
refraction,  a  heavenly  body  is  always  seen  nearer  the  zenith 
than  it  really  is.  The  refraction  is  small  near  the  zenith,  and 
at  an  altitude  of  45°  only  amounts  to  1',  a  quantity  that  the 
eye  could  barely  perceive.  But  it  increases  with  great  rapidity 
near  the  horizon,  where  it  amounts  to  more  than  half  a  degree. 
Hence,  on  a  level  plain,  or  at  sea,  we  still  see  the  setting  sun 
when  its  true  direction  is  below  the  horizon,  because  at  the 
horizon  the  refraction  is  a  little  greater  than  the  diameter  of 
the  sun.  In  this  case  the  lower  limb  of  the  sun  is  refracted 
more  than  the  upper  limb,  which  makes  the  sun  look  elliptical 
in  form,  the  horizontal  diameter  being  longer  than  the  vertical 
diameter. 

Another  consequence  is  that  the  sun  really  illuminates  a 
little  more  than  half  the  earth,  the  curvature  of  the  rays  bring- 
ing them  a  little  beyond  where  they  would  touch  the  earth  if 
there  were  no  atmosphere. 

Dispersion.  —  If  the  surface  where  the  ray  leaves  a  homo- 
geneous medium  is  parallel  to  that  by  which  it  enters,  as  in 
figure  26,  the  course  of  the  ray  after  leaving  will  be  parallel  to 
its  course  before  entering.  But  if  the  surfaces  are  not  parallel, 
this  will  not  be  the  case.  If  the  transparent  body  is  a  tri- 
angular prism,  as  shown  in  figure  29,  the  ray  may  be  refracted 
in  the  same  direction  both  on  entering  and  leaving.  In  this 
case  ordinary  light  will  not  leave  the  medium  as  a  single  ray, 
but  will  be  separated  into  rays  of  different  colors.  This  sepa- 
ration of  light  into  rays  of  different  colors  is  called  dispersion. 

The  effect  of  dispersion  is  seen  very  prettily  when  the  rays 
of  the  sun  are  passed  through  a  triangular  prism  of  flint  glass, 


OBSERVATION  AND  MEASUREMENT 


59 


and  thrown  upon  a  white  screen  or  wall.  We  may  then  dis- 
tinguish five  very  brilliant  colors,  red,  yellow,  green,  blue,  and 
violet,  as  well  as  some  intermediate  shades  between  these.  If 
we  notice  how  these  colors  are  placed,  we  shall  see  that  the 


FIG.  29.  —  Showing  how  rays  of  light  are  refracted  in  passing  through  a 

prism. 

red  light  is  refracted  from  its  course  the  least  of  all,  yellow 
more,  green  yet  more,  and  so  on.  This  shows  that  the  white 
light  of  the  sun  is  a  mixture  of  light  of  countless  different 
kinds,  each  kind  being  refracted  differently  from  the  other 
kinds. 

2.  Lenses  and  Object  Glasses.  —  When  rays  of  light  from  a 
distant  object  pass  through  a  convex  lens,  the  curvature  of  the 
surface  causes  the  rays  to  be  more  refracted  the  nearer  they 
pass  to  the  circumference  of  the  lens.  The  result  is  that  the 
rays  coming  from  any  one  point  of  the  object  all  converge  very 


FIG.  30.  —  Showing  how  parallel  rays  of  light  are  brought  to  a  focus  at  F 
by  passing  through  a  convex  lens.  An  observer  holding  his  eye  at 
F  and  looking  at  a  light,  however  small,  in  the  distance,  would  see 
the  whole  lens  illuminated  by  the  light. 

nearly  toward  a  certain  point,  ^(figure  30),  which  is  called  the 
focus  of  the  lens,  and  then  diverge  again  as  if  they  were  emitted 
by  the  focus.  The  effect  of  this  can  easily  be  seen  by  holding  a 


60 


ASTRONOMY 


common  reading  glass  or  magnifying  glass  perpendicular  to  a 
window  on  the  other  side  of  a  room.  If  you  then  hold  a  piece 
of  white  paper  at  the  proper  distance  beyond  the  glass,  you  will 
see  a  little  picture  of  the  window  on  the  paper.  A  picture  thus 
formed  by  a  lens  is  called  an  image  of  the  object  emitting  the 
light  that  forms  it.  The  lens  may  form  the  picture  in  the  air 
when  there  is  no  surface  on  which  the  light  may  fall. 

The  focal  length  of  a  lens  is  the  distance  from  its  center  to 
.the  image  of  a  distant  object  formed  by  it. 

The  ordinary  lenses  which  we  use  have  convex  surfaces  and 
are  called  convex  lenses.  But  a  lens  may  be  made  having  one 
or  both  surfaces  concave.  It  is  then  called  a  concave  lens. 


FIG.  81.  — Showing  how  rays  of  light  are  made  to  diverge  by  a  concave 
lens  instead  of  being  brought  to  a  focus. 

When  rays  from  an  object  fall  on  a  concave  lens,  they  are  not 
brought  to  a  focus,  but,  on  the  contrary,  are  made  to  diverge  by 
the  refraction  of  the  glass,  as  shown  in  figure  31. 

An  ordinary  lens  does  not  bring  all  the  light  actually  to  the 
same  focus,  on  account  of  the  dispersion  of  rays  of  different 
colors  just  described.  The  image  of  a  star,  instead  of  being 
a  point,  is  a  little  colored  circle  near  the  focus.  This  disper- 
sion is  called  chromatic  aberration,  and  results  in  indistinctness 
of  vision.  But  two  lenses  of  different  kinds  of  glass  may  be 
so  formed  that,  when  joined  together,  the  rays  passing  through 
them  shall  all  converge  almost  exactly  to  the  same  point.  One 
of  the  lenses  must  be  convex,  the  other  concave.  The  convex 


OBSERVATION  AND  MEASUREMENT 


61 


lens  is  commonly  made  of  crown  glass,  the  concave  one  of  flint. 
The  property  of  these  kinds  of  glass  is  that  flint  refracts  light 
about  as  much  as  crown,  but  disperses  the  rays  nearly  twice  as 
much.  The  dispersive  powers  of  the  concave  and  convex  glasses 
act  against  each  other,  so  that  the  rays  leave  the  last  lens  with- 
out dispersion  and  so  come  to  the  same  focus.  Such  a  com- 
bination is  called  achromatic  or  free  from  color  (figure  32). 


Axis    of 


Objective 


FIG.  32. — Section  of  an  achromatic  objective,  showing  the  form  of  the 
flint  and  crown  lenses.  The  crown  lens  is  always  convex  ;  the  flint 
has  at  least  one  surface  concave. 


3.  The  Refracting  Telescope.  —  A  refracting  telescope  is  one 
in  which  the  image  is  formed  by  a  lens  or  achromatic  combina- 
tion of  lenses  called  the  object  glass  or  objective  of  the  telescope. 
When  the  telescope  is  pointed  at  a  heavenly  body  or  other  dis- 
tant object,  the  rays  passing  through  the  object  glass  come  to  a 
focus,  and  form  an  image  of  the  object.  This  image  is  to  be 
seen  by  the  aid  of  an  eyepiece,  which  is  a  combination  of  two 
small  lenses  so  arranged  that  the  observer  can  get  as  good  a 
view  as  possible  of  the  image. 

Magnifying  Power.  —  The  magnifying  power  of  a  telescope 
is  the  number  of  times  that  it  makes  the  linear  dimensions  of 
an  object  seem  longer  than  they  do  to  the  naked  eye.  For 
example,  the  apparent  diameter  of  Jupiter  is  commonly  about 
40".  A  magnifying  power  of  50  would  make  it  appear  2000", 
or  more  than  33'  in  diameter,  and  larger  than  the  sun  or  moon. 

The  law  of  magnifying  power  is  that  it  is  equal  to  the  quo- 
tient of  the  focal  length  of  the  object  glass  divided  by  that  of  the 
eyepiece.  Thus,  with  a  telescope  of  eight  feet  focal  length  we 


62  ASTRONOMY 

should  get  a  magnifying  power  of  96  by  using  an  eyepiece  of 
one  inch  focus,  and  a  power  of  192  by  using  one  of  half  an 
inch  focus. 

It  follows  that,  with  any  telescope,  as  high  a  power  as  we 
wish  can  be  produced  by  using  an  eyepiece  small  enough.  But 
a  limit  is  soon  reached  beyond  which  a  higher  power  will  not 
enable  us  to  see  any  more,  because  the  light  becomes  fainter 
and  the  object  more  indistinct.  Commonly  an  eyepiece  between 
J  and  £  an  inch  in  focal  length  will  show  all  that  can  be  seen 
with  any  telescope. 

As  much  of  the  sky  as  we  can  see  in  a  telescope  at  one  time 
(as  magnified  in  the  telescope)  is  called  the  field  of  view  of  the 
telescope.  In  the  telescope,  the  field  of  view  commonly  looks 
very  large,  but  the  actual  portion  of  the  sky  which  it  takes  in  is 
very  small.  The  higher  the  magnifying  power,  the  smaller  it  is. 

The  line  which  forms  the  central  axis  of  the  tube  of  the  tele- 
scope, or  which  passes  through  the  centers  of  the  objective  and 
eyepiece,  is  directed  toward  the  center  of  the  field  of  view.  It 
is  called  the  line  of  sight  of  the  telescope. 

4.  The  Equatorial  Telescope.  —  If  we  point  a  telescope  at  a  star, 
and  do  not  move  it,  we  shall  see  the  star  move  rapidly  across 
the  field  of  view  and  disappear.  This  is  because  the  telescope 
stands  on  the  revolving  earth,  and  turns  with  it.  The  apparent 
diurnal  motion  of  a  star,  when  seen  in  a  telescope,  is  multiplied 
as  many  times  as  the  telescope  magnifies.  Hence  the  higher 
the  magnifying  power  of  a  telescope,  the  more  rapidly  the  star 
will  seem  to  move  across  the  field  of  view.  If  we  wish  the 
telescope  to  stay  pointed  at  a  star,  we  must  move  it  in  the 
opposite  direction  to  that  in  which  the  earth  turns.  This  is 
done  by  supporting  the  telescope  on  axes  on  which  it  can 
revolve.  The  machinery  by  which  the  telescope  is  made  to 
revolve,  and  the  handling  of  the  telescope  made  possible,  is 
called  the  mounting  of  the  telescope.  A  telescope  mounted  so 
as  to  follow  a  star  in  its  diurnal  motion  is  called  an  equatorial 
telescope,  or  simply  an  equatorial. 


OBSERVATION  AND  MEASUREMENT 


63 


Figure  33  shows  the  mounting  of  an  equatorial.  In  P  is  an 
axis  parallel  to  the  axis  of  the  earth,  the  upper  end  of  which, 
in  the  northern  hemisphere,  points  to  the  north  celestial  pole. 
This  axis  is  therefore  oblique  to  the  horizon.  It  is  called  the 
polar  axis  of  the 
telescope. 

To  the  upper 
end  of  the  polar 
axis  is  fastened 
a  sheath  D,  con- 
taining another 
axis.  This  is 
called  the  decli- 
nation axis,  be- 
cause by  turning 
the  telescope  on 
it,  the  latter  may 
be  pointed  at  any 
circle  of  declina- 
tion. 

By  turning  the 
telescope  on  these 
two  axes  we  can 
point  it  to  any 
part  of  the  heav- 
ens. If  we  wish 
it  to  stay  pointed 
at  a  star  with- 
out our  touching 
it,  the  telescope 
must  be  supplied 
with  a  clockwork 
so  made  as  to  keep  the  telescope  turning  from  east  toward 
west,  exactly  as  fast  as  the  earth  turns  from  west  toward  east. 
Then  by  pointing  the  telescope  at  a  star,  and  starting  the 
clockwork,  the  star  will  remain  in  the  field  of  view. 


FIG.  33.  — A  small  equatorial  telescope. 


64  ASTRONOMY 

5.  The  Reflecting  Telescope.  —  Rays  of  light  from  a  heavenly 
body  may  be  brought  to  a  focus  by  a  concave  mirror  as  well  as 
by  a  lens,  as  shown  in  figure  34.  On  passing  through  the  focus 


FIG.  34. — Showing  how  parallel  rays  falling  on  a  concave  mirror  are 
brought  to  a  focus  at  F. 

they  will  diverge  again,  as  they  do  after  passing  through  the 
focus  of  a  lens.  Hence  an  image  of  a  heavenly  body  may  be 
formed  in  the  focus  of  a  concave  mirror. 

A  reflecting  telescope  is  one  in  which  the  image  is  formed  by 
a  concave  mirror. 

Such  telescopes  can  be  made  of  larger  size  than  refracting 
telescopes,  but  they  are  not  so  convenient  to  use. 

The  observer,  to  view  the  image  directly,  would  have  to  stand 
in  front  of  the  mirror,  and  thus  be  in  the  way  of  the  light  from 
the  body  to  the  mirror.  The  best  way  of  avoiding  this  is  to 
put  a  small  diagonal  reflector  in  the  middle  of  the  tube,  nea- 
the  focus,  as  shown  in  figure  35.  Then  the  observer  looks  in 
sidewise  near  the  end  of  the  telescope  where  the  eye  is  shown 
in  the  figure.  The  small  mirror  and  its  supporting  piece  cut 
off  some  of  the  light,  but  not  so  much  as  the  observer's  head 
and  shoulders  would  cut  off  if  he  looked  directly  at  the  image. 

6.  Great  Telescopes.  —  Large  telescopes  are  objects  of  so  much 
interest,  that  a  short  history  of  their  growth  will  be  given. 
The  object  glasses  of  the  first  telescopes,  namely,  those  made 
by  Galileo  and  his  immediate  successors  between  1610  and 
1750,  consisted  of  only  a  single  lens.  Such  a  lens,  as  we  have 
already  seen,  refracts  the  light  of  different  colors  to  different 
foci.  For  this  reason  distinct  vision  was  impossible  with  these 


OBSERVATION  AND  MEASUREMENT 


65 


instruments.  Vision  was,  however,  im- 
proved by  making  them  very  long.  It  is 
said  that  some  were  100  feet  or  more  in 
length,  but  these  proved  to  be  quite  un- 
manageable and  were  probably 
of  very  little  use. 

This  difficulty  with  the  re- 
fracting telescope  led  Sir  Isaac 
Newton  to  propose  the  use  of  the  reflect- 
ing telescope,  which  was  free  from  chro- 
matic aberration.  He  made  some  small 
instruments  of  this  kind,  but  they  were 
little  more  than  toys  until  the  time  of  Sir 
William  Herschel.  This  great  astronomer 
was  at  the  height  of  activity  between  1770 
and  1800.  He  acquired  such  skill  in  mak- 
ing reflecting  telescopes  that  he  carried 
them  up  to  two  feet,  and  in  one  case,  four 
feet  in  diameter.  But  the  difficulty  was 
then  encountered  that  the  reflecting  mir- 
ror, when  large,  would  bend  under  its  own 
weight,  so  that  a  good  image  could  not  be 
formed.  Thus,  notwithstanding  the  celeb- 
rity of  Herschel's  great  40-foot  telescope, 
his  observations  were  nearly  all  made  with 
smaller  instruments. 

About  1760,  Dollond  of  London  invented 
the  achromatic  telescope.  In  this  instru- 
ment, as  previously  described,  the  object 
glass  is  composed  of  two  lenses  of  oppo- 
site curvatures  and  of  different  kinds  of 
glass.  But  it  was  long  found  impossible 
to  make  large  achromatic  telescopes,  owing 

to  the  difficulty  of    making   large   blocks 

£  a-   .  /  i.1.  A   FlG-  35-  — A  reflect- 

of  flint  glass  of  the  necessary  fineness  and      ing  teiescope  on  the 

purity.      This   kind   of    glass   contains   a      Newtonian  plan. 
NEWCOMB'S  ASTRON.  —  5 


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66  ASTRONOMY 

large  quantity  of  lead,  and  the  lead  would  sink  down  to  the 
bottom  of  the  pot  in  which  the  glass  was  melted,  and  thus 
make  the  glass  heterogeneous  and  unfit  for  use.  Thus,  a  hun- 
dred years  ago,  a  refracting  telescope  four  inches  in  diameter 
was  considered  large. 

About  1810,  Guinand,  a  Swiss  glass  maker,  found  a  method 
of  making  disks  of  glass  much  larger  than  had  before  been 
possible.  At  the  same  time  rose  the  celebrated  Fraunhofer,  a 
German  optician,  who  acquired  remarkable  skill  in  grinding 
and  figuring  the  lenses  of  object  glasses  into  exact  shape.  He 
and  his  successors  in  Germany  carried  refracting  telescopes  up 
to  15  inches'  aperture.  In  1845  a  telescope  of  this  size  was 
made  for  the  Harvard  Observatory  in  Cambridge,  Massachu- 
setts, and  became,  in  consequence  of  its  size  and  excellence, 
one  of  the  celebrated  instruments  of  the  world. 

About  the  same  time,  Lord  Rosse  of  Ireland  made  his  cele- 
brated reflecting  telescope,  six  feet  in  diameter.  This  is  still, 
in  size,  the  greatest  telescope  ever  constructed.  But  the  impos- 
sibility of  keeping  the  mirror  in  proper  polish  and  preventing 
it  from  bending  under  its  own  weight  is  such  that  more  can  be 
seen  with  smaller  telescopes  of  different  construction  than  with 
this  celebrated  instrument.  Hence,  it  became  desirable  to 
improve  the  refracting  telescope  still  farther. 

The  first  one  to  improve  on  the  work  of  Fraunhofer  and  his 
successors  was  an  American,  Alvan  Clark,  of  Cambridgeport, 
Massachusetts.  After  making  a  number  of  small  telescopes 
whose  object  glasses  proved  to  be  superior  to  any  ever  before 
figured,  he  succeeded,  about  1861,  in  making  a  telescope  of  18 
inches'  aperture,  which  was  then  the  largest  ever  made.  This 
instrument  still  exists  at  the  observatory  of  the  Northwestern 
University,  Evanston,  Illinois. 

Shortly  afterward,  Cooke,  of  England,  made  a  telescope  of 
25  inches'  aperture,  which  was  therefore  much  larger  than 
that  of  Clark.  This  telescope  was  made  for  Mr.  Newall  of 
England.  It  is  now  at  the  University  of  Cambridge. 

In  1873  Clark  finished  two  telescopes  an  inch  larger  than 


OBSERVATION  AND  MEASUREMENT 


67 


that  of  Mr.  Newall.  One  was  for  the  Naval  Observatory 
of  Washington,  the  other  was  given  by  its  owner,  Mr.  L. 
McCormick,  to  the  University  of  Virginia. 


FIG.  36.  —  Great  40-inch  telescope  of  the  Yerkes  Observatory. 
Next,  a  telescope  of  30  inches'  aperture  was  made  in  France 
by  the  Brothers  Henry,  and  is  now  mounted  at  the  observatory 
of  Nice  011  the  coast  of  the  Mediterranean. 


68  ASTRONOMY 

In  1883  Mr.  Clark  and  his  two  sons  made  the  object  glass 
of  another  telescope  of  30  inches7  aperture  for  the  observatory 
at  Pulkowa,  in  Russia.  The  mounting  of  this  instrument  was 
made  by  the  Repsolds  of  Hamburg. 

In  1876  Mr.  James  Lick,  of  California,  gave  money  to  found 
an  observatory,  which  was  to  be  provided  with  the  largest 
telescope  that  had  ever  been  constructed.  The  work  of  making 
the  object  glass  of  the  instrument  was  again  intrusted  to 
Messrs.  Alvan  Clark  and  Sons,  but  great  difficulty  was  found 
in  getting  disks  of  glass  of  the  necessary  size  and  purity.  At 
length,  after  many  years  of  failure,  a  Frenchman  succeeded 
in  the  difficult  task  of  making  excellent  disks  of  36  inches' 
diameter.  With  these  the  Messrs.  Clark  completely  finished 
the  object  glass  of  the  telescope  in  the  year  1887.  The  mount- 
ing was  made  by  Warner  and  Swasey,  of  Cleveland,  Ohio. 
Mr.  Lick's  observatory,  which  is  called  after  him,  was  built 
on  Mount  Hamilton,  in  California,  and  the  telescope  com- 
menced its  work  there  in  1888. 

The  largest  refracting  telescope  now  in  actual  use  is  that 
built  at  the  expense  of  Mr.  Yerkes  of  Chicago  for  the  univer- 
sity of  that  city.  The  object  glass  is  40  inches  in  diameter, 
and  was  figured  by  Alvan  G.  Clark,  the  son,  and  mounted  by 
Warner  and  Swasey.  The  Yerkes  Observatory,  in  which  it  is 
placed,  is  near  the  shore  of  Lake  Geneva,  Wisconsin. 

7.  Meridian  Instruments. — A  telescope  of  some  sort  is  an 
essential  part  of  every  instrument  intended  for  exact  astro- 
nomical observation  and  measurement.  One  of  the  most 
common  of  astronomical  instruments  is  the  meridian  transit 
instrument.  Instead  of  being  mounted  like  an  equatorial,  so 
as  to  be  pointed  in  any  direction,  it  turns  on  only  a  single 
horizontal  axis,  having  an  east  and  west  direction.  Thus  the 
telescope  turns  only  in  the  plane  of  the  meridian,  so  that  it 
will  show  us  objects  only  while  they  are  crossing  the  meridian. 

To  explain  the  use  of  the  transit  instrument  we  must  recall 
what  we  have  said  about  sidereal  time.  A  sidereal  clock  is  set 


OBSERVATION  AND  MEASUREMENT 


69 


running  in  such  a  way  that  its  hands  shall  point  at  Oh.  Om.  Os. 
when  the  vernal  equinox  is  crossing  the  meridian.  Then  as 
the  various  heavenly  bodies  are  seen  in  the  transit  instrument, 
crossing  the  meridian,  the  time  shown  by  the  hands  on  the 
face  of  the  sidereal  clock  shows  the  right  ascension  of  each. 

If  you  should  look  into  a  transit  instrument,  you  would  see 
one  or  more  dark  lines  passing  up  and  down  across  the  field  of 
view.  These  are  fine  lines  made 
of  spider  web,  the  middle  one  of 
which  marks  the  meridian.  The 
moment  at  which  a  star  crosses 
this  line  may  be  noted  on  the 
clock  within  a  small  fraction  of 
a  second.  This  gives  us  the  right 
ascension  of  the  star  with  the 
same  precision. 

This  instrument  also  enables  us 
to  determine  the  time  of  day,  or 
the  error  of  a  clock  or  watch,  with 
the  same  exactness.  The  observer 
notes  the  time  by  the  clock  at 
which  a  star  of  known  right  as- 
cension crosses  the  meridian ;  the  difference  between  the  clock 
time  and  the  right  ascension  is  the  error  of  his  clock,  for  which 
he  can  make  due  allowance  at  any  moment. 

The  moment  of  noon  is  sent  out  by  a  telegraphic  signal 
from  different  observatories  to  railway  offices  and  elsewhere, 
so  that  any  one  who  receives  the  signal  may  set  his  clock  exact 
to  a  second,  if  he  has  the  skill  to  do  it  and  exercises  the 
necessary  care. 

To  the  transit  instrument  are  sometimes  attached  vertical 
circles,  which  will  turn  with  the  instrument.  These  circles 
have  fine  lines  engraved  all  round  their  circumference,  so  as  to 
mark  off  the  degrees  and  minutes  of  the  circle.  By  their  use 
the  declination  of  a  star,  as  it  passes  the  meridian,  may  be 
observed. 


FIG.  37.  _  The  threads  in  the 
focus  of  a  transit  instru- 
ment, with  a  star  passing 
over  them. 


ASTRONOMY 


FIG  38  -A  meridian  circle  seen  from  the  south.     O,  C  are  the  graduated 
'circles  which  are  divided  into  degrees  and  small  fractions  of  a  degree 
generally  two  or  five  minutes.     M,  M  are  four  microscopes  through 
which  the  graduations  on  these  circles  are  read. 


OBSERVATION  ANE  MEASUREMENT  71 

An  instrument  moving  in  the  meridian,  and  provided  with 
such  a  circle,  so  that  both  right  ascension  and  declination  may 
be  observed,  is  called  a  meridian  circle. 

8.  The  Spectroscope  and  its  Use.  —  We  have  seen  that  ordi- 
nary light,  which  seems  to  us  white,  is  made  up  of  countless 
rays  of  different  colors,  which  can  be  seen  separately  by 
passing  them  through  a  prism,  which  causes  them  to  be 
dispersed. 

When  the  dispersed  rays  fall  on  a  surface  so  that  the 
different  colors  can  be  seen,  the  appearance  is  called  a 
spectrum. 

The  solar  spectrum  is  the  spectrum  which  is  formed  when 
the  rays  of  the  sun  are  dispersed  in  the  way  described. 

The  spectrum  of  a  star,  a  planet,  or  any  other  body,  is  the 
spectrum  produced  by  the  light  coming  from  that  body. 

Different  substances  produce  different  kinds  of  spectra,  so 
that  it  is  frequently  possible,  from  the  nature  of  a  spectrum, 
to  know  what  kind  of  substance  emitted  the  light,  or  through 
what  kind  of  transparent  medium  the  light  has  passed.  The 
operation  of  detecting  substances  by  their  spectra  is  called 
spectrum  analysis. 

The  Spectroscope.  —  A  spectroscope  is  any  instrument  for 
showing  or  photographing  a  spectrum.  The  simplest  spec- 
troscope of  all  consists  of  a  triangular  prism  of  flint  glass, 
like  that  of  which  the  section  is  shown  in  figure  29.  If  we 
look  through  such  a  prism  at  a  bright  star  or  a  distant  light, 
we  shall  see  the  object  spread  out  into  a  line  of  light  of  which 
the  color  varies  from  red  at  one  end  to  blue  or  violet  at  the 
other,  just  as  when  the  light  of  the  sun  passing  through  such 
a  prism  is  thrown  on  a  screen.  When  we  look  at  the  object 
directly,  the  retina  of  the  eye  is  the  screen  on  which  the, 
image  falls. 

The  simplest  kind  of  an  astronomical  spectroscope  is  shown 
in  figure  39.  This  one  is  used  in  connection  with  a  telescope. 
At  S  is  a  very  narrow  slit,  between  two  plates  of  metal  shown 


72 


ASTRONOMY 


in  figure  40.     This  is  fastened  to  the  eye  end  of  a  large 
telescope  so  that  the  slit  shall  be  in  the  focus.     This  telescope, 


FIG.  39. 

not  shown  in  the  figure,  is  pointed 
of  the  star  shall  be  formed  in  the 


FIG.  40.  —  The  slit  of  a  spectroscope.  AB 
and  CD  are  slides  in  a  plate  of  metal 
having  an  opening  in  it,  shown  by  the 
dotted  lines.  This  opening  is  nearly 
covered  by  the  two  plates  of  metal  K 
and  L,  which  move  in  the  slides  by  a 
screw,  and  may  be  adjusted  so  as  to 
form  a  slit  SS  as  narrow  as  we  please. 
This  slit  is  placed  in  the  focus  of  the 
telescope. 


at  a  star  so  that  the  image 
slit  through  which  all  the 
rays  from  it  will  pass. 
Then  the  rays  diverge  as 
shown  by  the  dotted  lines 
and  pass  through  an  ob- 
ject glass  A.  The  focus 
being  at  the  slit,  the  rays 
of  any  color,  yellow  for 
example,  will,  after  pass- 
ing through  the  object 
glass,  -be  refracted  so  as 
to  be  parallel  to  each 
other. 

This  combination  of  the 
object  glass  A  with  the 
slit  in  its  focus  is  called 
the  collimator  of  the  spec- 
troscope. 

After  leaving  the  colli- 
mator the  rays  next  fall 
on  the  prism  P,  by  which 


OBSERVATION  AND  MEASUREMENT  73 

they  are  refracted  in  the  way  already  shown.  Next  they  pass 
to  a  second  object  glass  B,  which  is  part  of  a  small  telescope, 
and  are  again  brought  to  a  focus  at  C. 

The  action  of  the  whole  apparatus  is  such  that  rays  of  the 
same  color  come  to  the  same  focus  at  (7,  while  those  of  a 
different  color  will  come  to  a  focus  above  or  below  the  others, 
according  to  their  different  colors. 

Then,  an  observer  looking  into  C  with  an  eyepiece  E,  as  in 
an  ordinary  telescope,  will  see  the-  spectrum  of  the  star. 

If,  instead  of  using  the  eye,  a  sensitized  photographic  plate 
is  inserted  at  (7,  a  photograph  of  the  spectrum  may  be  taken. 
But  this  photograph  will  not,  of  course, 
show  the  different  colors  of  the  light. 

In  a  powerful  spectroscope,  instead  of 
a  single  prism  at  P,  a  long  row  of  prisms 
is  used  in  order  to  secure  a  greater  dis- 
persion of  the  light.  FIG.  41.—  The  objective 

A   yet    simpler   form   of   spectroscope      spectroscope  consist, 

.  ,        ,.        ,  ,  .        ,  ,   ,  ,        mg  of  an  object  glass, 

consists  of  nothing  but  a  telescope  and      with     a     refracting 

a  prism,  the  latter  being  put  over  the  prism  in  front  of  it, 
object  glass  as  shown  in  figure  41.  Then  by  which  the  light 


an   observer   looking   into  the   telescope      of  a  *tar  ™y  bef  dis~ 

persed  before  it  en- 
will  see  the  spectrum  of  the  star.    A  pho-      ters  tne  telescope. 

tograph  of  this  spectrum  may  be  taken 
in  the  same  way  as  with  the  telescope.     With  such  an  instru- 
ment photographs  of  the  spectra  of  all  the  stars  in  the  field 
of  view  of  the  telescope  may  be  made. 

Kinds  of  Spectra.  —  If,  with  any  form  of  spectroscope,  we 
look  at  the  flame  of  a  candle  or  any  other  artificial  light,  not 
very  far  away,  the  spectrum  will  show  the  whole  series  of 
spectral  colors  without  any  dark  lines;  but  if  the  light  at 
which  we  look  is  several"  miles  away,  a  distant  gaslight,  for 
example,  we  shall  see  that  the  spectrum  is  crossed  by  a 
number  of  dark  lines.  Why  do  these  lines  appear  when  the 
light  is  at  a  distance,  and  not  when  the  light  is  near  ?  Experi- 
ments show  that  it  is  because  some  of  the  light  is  absorbed  by 


74 


ASTRONOMY 


[indigo 


(Green 


'ellow 


I  Orange 


Red 


FIG.  42. 

The  solar  spectrum  with 
its  dark  lines. 


the  air  through  which  it  passes.  The 
light  absorbed  is  that  which  belongs 
in  the  place  of  the  dark  lines.  The 
process  by  which  this  is  brought  about 
is  called  selective  absolution.  The  word 
selective  implies  that  the  air  does  not 
absorb  all  the  different  kinds  of  light 
equally,  but  picks  out,  or  selects,  cer- 
tain'kinds. 

Spectrum  of  the  Sun.  —  If,  instead 
of  looking  at  a  distant  light,  we  ex- 
amine the  light  of  the  sun  by  the 
spectroscope,  we  shall  find  that,  in  ad- 
dition to  the  lines  produced  by  the  air, 
there  are  a  great  number  of  other 
dark  lines  in  the  spectrum.  These  are 
formed  by  the  selective  absorption  of 
the  sun's  atmosphere,  which  is  dif- 
ferent from  the  atmosphere  of  the 
earth. 

If,  instead  of  the  sun,  we  examine 
the  spectra  of  the  stars,  we  shall  find 
yet  different  lines.  If  we  examine 
the  spectra  of  some  kinds  of  burning 
gas,  we  shall  find  one  or  more  bright 
lines  varying  according  to  the  nature 
of  the  gas.  This  shows  that  such 
a  gas  does  not  give  out  light  of  all 
colors,  but  only  light  of  particular 
colors. 

9.   Semidiameter   and   Parallax.  —  If 

an  observer  with  his  eye  at  E,  figure 
43,  is  looking  at  a  globe,  as  AB,  the 
apparent  diameter  of  this  globe,  as  it 
appears  to  him?  will  be  the  angle  be- 


OBSERVATION  AND  MEASUREMENT 


76 


tween  the  lines  EA  and  EB  drawn  from  his  eye  so  as  to 
touch  the  globe.  One  half  of  this  diameter,  or  either  of  the 
angles  AEG  or  BEC,  is  called  the  semidiameter.  Thus  by  the 


FIG.  43. — Apparent  diameter  of  the  sun,  moon,  or  other  heavenly  body, 
as  seen  by  an  observer  at  E.  The  diameter  is  the  angle  AEB,  sub- 
tended by  the  whole  diameter  of  the  body,  while  the  semidiam- 
eter is  the  angle  CEA  or  CEB  between  the  center  and  apparent 
circumference. 

semidiameter  of  the  sun  or  moon  we  mean  the  angle  between  tf 
two  lines,  one  of  which  is  drawn  to  the  center  of  the  sun  orf' 
moon,  and  the  other  to  its  apparent  circumference. 

It   is  evident  that   the   semidiameter   of  a  given   body  is 
smaller,  the  farther  the  body  is  away. 

The  parallax  of  a  heavenly  body  is  the  difference  of  the  // 
directions  in  which  it  is  seen  from  two  different  points. 

Let  S,  figure  44,  be  the  body,  and  A  and  B  the  two  points 
from  which  it  is  seen. 


FIG.  44.  —  Showing  the  parallax  of  a  body  at  8. 

An  observer  at  A  looking  at  8  will  see  it  in  the  direction 
ASX.  An  observer  at  B  will  see  it  in  the  direction  BST. 
The  difference  of  these  directions  is  the  angle  XSY,  which  is 
equal  to  the  angle  A/SB.  This  angle  is  the  parallax  of  the  body 
for  these  two  observers.  In  measuring  parallax  we  may  sup- 
pose the  lines  SX  and  S  Y  continued  till  they  reach  the  celes- 
tial sphere.  The  parallax  is  then  an  arc  XYou  the  sphere. 


76  ASTRONOMY 

Since  the  heavenly  bodies  are  seen  by  observers  from  differ- 
ent points  of  the  earth,  they  all  have  parallaxes.  When  an 
astronomer  wishes  to  compute  the  direction  of  such  a  body 
from  where  he  is  stationed  at  a  certain  time,  he  first  computes 
its  direction  from  the  center  of  the  earth.  Then  he  computes 
the  difference  of  the  direction  from  the  center  of  the  earth  and 
from  his  point  of  observation.  Since  he  is  carried  around  by 
the  turning  of  the  earth  on  its  axis,  it  follows  that  the  paral- 
lax will  be  continually  changing  on  account  of  this  motion. 

The  horizontal  parallax  of  a  body  is  the  difference  of  its 
direction  AS  (figure  45)  when  it  lies  in  the  horizon  of  an  ob- 
server at  A,  and  the  direction  OS  in  which  it  lies  from  the 
center  of  the  earth.  We  see  that  this  angle  is  the  same 
as  the  semidiameter,  ASC,  of  the  earth  as  it  would  be  if  seen 
from  the  body  S. 


FIG.  46.  — Horizontal  parallax  of  a  body  at  8.    The  circle  represents 

the  earth. 

It  is  evident  that  the  horizontal  parallax  of  a  body  is  less, 
the  greater  the  distance  of  the  body.  Hence  the  heavenly 
bodies  have  a  less  or  greater  parallax  according  to  their  greater 
or  less  distances.  Thus  parallax  and  distance  stand  in  a  cer- 
tain relation  to  each  other. 

The  moon  being  the  nearest  body  to  the  earth,  it  has  the 
greatest  parallax  of  all  the  heavenly  bodies.  Its  horizontal 
parallax  is  generally  about  one  degree.  This  is  nearly  twice 
its  apparent  diameter. 

If  an  observer  in  New  York,  looking  at  the  moon  when  near 
the  meridian,  should  see  her  just  south  of  a  star,  one  looking 
at  her  in  Chile  would  at  the  same  moment  see  her  north  of  the 
star. 


OBSERVATION  AND  MEASUREMENT  77 

The  parallax  of  the  other  bodies  of  the  solar  system  is  so 
small  that  the  eye  could  not  perceive  it.  The  sun's  horizontal 
parallax  is  about  8.8".  This  is  the  same  thing  as  saying  that 
the  earth  would  have  an  apparent  semicliameter  8.8",  if  we  could 
view  it  from  the  sun.  Although  a  body  of  this  size  would  be 
invisible  to  the  naked  eye,  it  would  be  a  large  object  when 
measured  with  the  telescope. 

The  only  way  in  which  the  distances  of  the  bodies  of  the 
solar  system  can  be  directly  measured  is  by  their  parallax. 
Two  observers  on  opposite  sides  of  the  earth,  making  exact 
observations  of  the  direction  in  which  they  see  the  moon  or  a 
planet,  can  determine  the  parallax  of  the  body  observed,  and 
from  that  can  learn  its  distance.  Thus,  to  express  the  distance 
of  the  sun,  astronomers  say  that  its  horizontal  parallax  is  8.8". 

There  are,  however,  other  ways  of  getting  at  distances  in 
the  solar  system.  For  example,  the  distance  of  the  moon  is 
determined  by  calculating  how  far  off  it  must  be  to  revolve 
around  the  earth  in  the  time  we  see  that  it  actually  does 
revolve. 

The  distance  of  the  sun  is  also  determined  by  the  velocity 
of  light.  It  is  found  by  experiment  that  light  travels  about 
186,000  miles  in  a  second.  It  is  also  found  that  it  takes  500 
seconds  for  light  to  travel  from  the  sun  to  the  earth.  It  is  a 
very  simple  problem  to  find  from  these  figures  how  far  the  sun 
must  be. 

10.  The  Aberration  of  Light. — It  is  found  by  very  exact 
astronomical  observations  that  we  do  not  see  a  star  in  its  true 
direction  unless  it  lies  in  the  direction  of  the  earth's  motion 
round  the  sun  at  the  moment  of  observation.  In  all  other 
positions  the  star  will  seem  to  be  displaced  in  the  direction 
toward  which  the  earth  carrying  the  observer  is  moving  at  the 
moment.  For  example,  if  the  star  is  in  the  position  S,  figure 
46,  and  the  earth  at  E  is  moving  in  the  direction  of  the  arrow, 
then  the  star  will  appear  as  in  the  position  T,  in  the  direction 
shown  by  the  dotted  line.  This  displacement  arises  from  the 


78 


ASTRONOMY 


combination  of  the  earth's  motion  with  the  motion  of  light.  It 
is  called  the  aberration  of  light,  or,  for  shortness,  aberration 
simply. 


FIG.  46. 


FIG.  47. 


To  explain  aberration  suppose  AB,  figure  47,  to  be  a  very 
long  and  narrow  tube,  and  let  the  dotted  line  be  a  ray  of  light 
from  a  star,  so  that,  if  the  tube  were  at  rest,  the  ray  would 
pass  centrally  through  it,  from  A  to  B.  Then  an  observer 
looking  through  the  tube  at  B  would  see  the  star  in  the  central 
line  of  the  tube. 

Now  suppose  the  tube  and  observer  to  be  moving  in  the 
direction  shown  by  the  arrow,  so  that  while  the  light  is  pass- 
ing from  A  to  J5,  the  tube  is  carried  from  the  position  AB  to 
the  position  CD.  Then,  the  motion  of  the  tube  would  cause 
the  ray  to  strike  the  side  of  the  tube  before  getting  through  it, 
so  that  the  observer  would  not  see  the  star. 

In  order  that  the  star  may  be  seen  while  the  tube  is  in 
motion,  the  latter  must  be  inclined  in  the  position  AN.  Then, 


OBSERVATION  AND  MEASUREMENT  79 

while  the  ray  of  light  is  passing  from  A  to  JB,  the  end  N  of 
the  tube  will  be  carried  from  N  to  B,  and,  in  consequence,  the 
light  will  pass  centrally  through  the  tube.  Hence  the  observer, 
looking  in  at  JV,  will  now  see  the  star  in  the  direction  NA  in- 
stead of  the  true  direction  BA. 

The  velocity  of  light  is  very  nearly  10,000  times  that  of  the 
earth  in  its  orbit.  Hence  the  length  AB  is  about  10,000  times 
the  space  NB  through  which  the  tube  moves  while  the  light  is 
passing  through  it.  The  corresponding  angle  NAB  is  called 
the  constant  of  aberration.  Its  amount  is  about  20.5". 

There  are  many  every-day  phenomena  depending  on  the 
same  principle  that  the  aberration  of  light  does.  If  a  steamer 
while  in  motion  has  a  side  wind  blowing  against  her,  the  wind 
will  seem  to  the  passengers  to  blow  from  a  point  nearer  the 
direction  in  which  the  ship  is  going  than  the  one  from  which 
it  really  blows. 

If  one  drives  rapidly  through  a  shower  of  rain  falling 
straight  down,  and  watches  the  direction  of  the  motion  of  the 
drops,  they  will  be  seen  falling  obliquely  as  if  carried  back- 
ward by  a  wind. 


CHAPTER  V 
GRAVITATION 

1.  Force.  —  The  motions  of  the  heavenly  bodies  seem  so 
different  from  the  motions  we  are  accustomed  to  see  on  the 
earth,  that  for  many  generations  it  was  supposed  that  they 
could  not  be  explained  by  the  same  laws.  We  have  now  to 
see  how  it  is  that  the  planets  revolve  around  the  sun  accord- 
ing to  the  same  laws  which  govern  the  motion  of  a  ball  thrown 
into  the  air.  To  do  this,  we  must  learn  the  meaning  of  certain 
words. 

The  substance  of  anything  we  can  see  or  feel  is  called 
matter. 

Anything  made  up  of  matter,  and  considered  as  a  thing  by 
itself,  is  called  a  body.  For  example,  a  ball  is  a  body;  the 
rubber,  leather,  and  yarn  of  which  it  is  made  are  matter. 

That  which  makes  a  body  move  or  stop  moving  is  called 
force. 

For  example,  if  you  throw  a  ball,  your  hand  exerts  a  force 
on  the  ball ;  it  is  that  force  which  sets  the  ball  in  motion.  As 
the  ball  flies  through  the  air,  the  air  exerts  a  force  against  it ; 
this  force  makes  it  go  slower  than  it  otherwise  would  go. 
When  the  ball  strikes  the  ground,  the  ground  exerts  a  force 
against  it ;  this  force  soon  stops  it. 

Friction  is  a  force  exerted  by  one  body  upon  another  that 
rubs  against  it.  If  you  try  to  draw  a  sled  on  ice,  you  can  pull 
it  along  very  easily.  But  if  you  try  to  draw  the  same  sled 
with  the  same  load  on  a  smooth  pavement,  you  will  have  to 
pull  harder,  and  on  a  rough  pavement  yet  harder.  This  is 

80 


GRAVITATION  81 

because  the  pavement  exerts  a  greater  friction  against  the 
runners  of  the  sled  than  the  ice  does. 

Gravity  is  the  force  by  which  all  bodies  on  the  earth  tend  to 
fall  toward  the  center  of  the  earth.  This  force  is  familiar  to 
all  of  us  from  infancy.  Every  child  that  falls  down  and  every 
stone  that  lies  on  the  ground  do  so  because  of  this  force.  If 
it  did  not  exist,  we  could  not  keep  ourselves  or  any  loose  object 
from  slowly  flying  away  from  the  earth  except  by  fastening  it 
down. 

2.  The  Laws  of  Motion.  —  By  long  study  and  experiment  men 
have  learned  that  there  are  certain  laws  according  to  which  all 
bodies  move.  These  are  called  Newton's  laws  of  motion.  The 
first  of  these  laws  is  this :  — 

Every  body  in  motion,  and  not  acted  upon  by  any  force,  will  move 
forward  in  a  straight  line  with  unchanging  velocity  forever. 

It  took  men  a  long  time  to  find  out  this  law,  because  no 
unsupported  body  on  the  surface  of  the  earth  ever  moves  in  a 
straight  line,  or  continues  moving  forever.  The  reason  is  that 
all  moving  bodies  around  us  are  acted  on  by  forces,  in  spite  of 
all  we  can  do  to  prevent  their  action. 

One  of  these  forces  is  that  of  gravity,  which  will  always 
bring  a  body  down  to  the  earth  if  it  is  not  supported.  Another 
is  friction.  If  a  body  is  thrown  through  the  air,  the  friction 
and  resistance  of  the  air  are  forces  tending  to  stop  it. 

The  less  the  friction,  the  farther  a  body  will  move.  On  a 
railway  the  friction  is  slight,  so  that  a  train  will  run  some 
distance  even  if  the  locomotive  leaves  it.  If  there  were  no 
friction  or  resistance  at  all,  it  would  run  to  the  end  of  the  road 
all  by  itself. 

When  a  body  is  not  held  in  any  way,  it  is  said  to  be  free  to 
move.  No  bodies  on  the  surface  of  the  earth  are  perfectly  free 
to  move  unless  they  are  thrown  into  the  air,  because,  when 
they  rest  on  the  ground,  there  is  always  friction  which  hinders 
their  motion.  There  is  a  little  friction  even  if  they  float  in 
NEWCOMB'S  ASTRON.  — 6 


82  ASTRONOMY 

water  \  and  this  friction  will  increase  when  the  body  is  set  in 
motion.     But  the  heavenly  bodies,  not  resting  on  or  touching 

anything,  are  perfectly  free  to  move. 
The  second  law  of  motion  is  this ;  — 

When  a  force  acts  on  a  body  free  to  move,  it  takes  tune  for  the 
force  to  produce  a  change  in  the  motion  of  the  body,  and  the  greater 
the  force,  and  the  greater  time  during  which  it  acts  on  the  body,  the 
greater  the  change  of  motion  of  the  body. 

Every  one  must  have  noticed  this  when  a  train  is  starting 
from  a  station.  When  the  engine  first  pulls,  the  motion  is  so 
slow  that  we  hardly  notice  it.  In  a  few  seconds  the  train  is 
going  faster,  and  in  a  few  seconds  more,  yet  faster,  the  engine 
pulling  its  utmost  all  the  time.  A  minute  or  more  may  be 
required  before  the  strongest  pull  of  the  engine  will  get  the 
train  up  to  full  speed.  Then  as  people  sometimes  learn  to 
their  sorrow,  it  takes  time  for  any  force  to  stop  the  train. 
The  engineer  may  apply  his  brakes  with  all  their  force,  and 
yet  the  train  move  a  quarter  of  a  mile  before  stopping. 

This  property  of  matter  by  which  time  is  required  for  a 
force  to  set  it  in  motion,  and  again  time  is  required  for  the 
force  to  stop  it,  is  called  inertia. 

The  third  law  of  motion  is  this :  — 

Whenever  one  body  exerts  a  force  on  another,  the  latter  exerts  an 
equal  and  opposite  force  on  it. 

Slap  a  wall  with  your  hand,  and  you  will  find  that  the  wall 
slaps  your  hand  as  sharply  as  your  hand  does  the  wall.  When 
the  wheels  of  an  engine  begin  to  turn,  all  the  steam  does  is  to 
make  the  wheels  exert  a  force  on  the  track.  In  doing  this, 
the  track  exerts  an  equal  force  on  the  wheels,  and  this  makes 
the  engine  move  forward.  When  men  floating  on  a  raft  in  a 
shallow  river  push  the  bottom  with  their  poles,  the  bottom 
pushes  the  pole,  the  pole  the  men,  and  the  men  the  raft.  This 
opposite  force  is  called  reaction,  and  the  original  force  is  called 
the  action.  Thus  action  and  reaction  are  always  equal,  and  in 
opposite  directions. 


GRAVITATION  83 

3.   Universal    Gravitation.  —  Everything  we    see   about  us 

tends  to  fall  downward  toward  the  center  of  the  earth.  This 
is  because  the  matter  of  the  earth  attracts  everything  on  it 
toward  the  center  This  attraction,  or  the  tendency  of  things 
to  fall  downward,  is  called  gravitation,  and  has  always  been 
known  to  men.  What  was  not  known  till  modern  times  is 
that  all  the  heavenly  bodies,  sun,  planets,  and  stars,  also  attract 
other  bodies  toward  their  centers.  The  general  fact  of  this 
attraction  is  called  universal  gravitation. 

When  it  was  well  understood  that  the  planets  revolved 
around  the  sun,  it  was  still  difficult  for  men  to  understand  the 
laws  according  to  which  they  moved.  It  was  evident  that 
there  must  be  some  cause  connecting  their  motions  with  the 
sun.  But  when  the  laws  of  motion  were  not  known,  it  was 
impossible  to  say  exactly  what  that  cause  was.  Between  the 
years  1600  and  1680,  it  began  to  be  suspected  that  there  was 
some  attraction  between  the  sun  and  the  planets.  Sir  Isaac 
Newton  about  the  year  1680  was  the  first  to  prove  this,  and  to 
lay  down  the  laws  of  motion  with  such  clearness  and  exact- 
ness that  no  doubt  could  remain  on  the  subject.  The  law  of 
universal  gravitation,  called  Newton's  law,  is  this :  — 

Every  particle  of  matter  in  the  universe  attracts  every  other  par- 
ticle with  a  force  which  varies  directly  as  the  masses  of  the  particles 
and  inversely  as  the  square  of  their  distance  from  each  other. 

This  is  also  called  the  law  of  the  inverse  square.  It  may  be 
understood  in  this  way  :  Twice  as  far  away,  the  attraction  is 
one  fourth  as  much,  4  being  the  square  of  2 ;  three  times  as 
far  away,  one  ninth  as  much,  9  being  the  square  of  3 ;  and  so 
on.  The  distance  of  the  moon  is  about  60  times  the  radius  of 
earth ;  the  square  of  60  is  3600.  Therefore,  anything  at  the 
distance  of  the  moon  will  be  attracted  toward  the  earth's 
center  about  -^Vff  part  as  much  as  it  would  at  the  earth's 
surface. 

It  follows  that  the  higher  up  we  carry  any  body,  the  lighter 
it  is.  It  is  true  that  even  in  a  balloon,  the  change  of  weight  is 


84  ASTRONOMY 

not  sensible  to  ordinary  observation.  But  at  the  international 
office  of  weights  and  measures,  near  Paris,  weighing  has  been 
brought  to  such  perfection  that,  when  one  weight  is  laid  on  top 
of  another  in  the  scale  pan,  the  combined  weight  of  the  two  is 
found  to  be  less  than  when  they  are  laid  side  by  side.  The 
reason  is  that  the  upper  weight  is  farther  from  the  center  of 
the  earth  when  on  top  of  the  other  than  when  lying  along- 
side of  it,  and  therefore  is  lighter. 

The  third  law  of  motion  applies  to  gravitation.  Whenever 
one  body  attracts  another,  the  other  attracts  it  equally  in 
return.  A  stone  attracts  the  earth  as  much  as  the  earth  does 
the  stone.  A  planet  attracts  the  sun  as  much  as  the  sun  does 
the  planet. 

4.  Weight  and  Mass.  —  The  weight  of  a  body  is  the  force 
with  which  it  is  attracted  by  the  earth. 

The  mass  of  a  body  is  the  quantity  of  matter  which  it 
contains. 

The  mass  of  a  body  is  measured  by  its  inertia,  or  by  the 
force  required  to  give  it  a  certain  motion  in  a  certain  time. 

In  ordinary  life,  mass  and  weight,  at  any  place,  are  always 
in  the  same  proportion  to  each  other,  so  that  we  need  not  make 
any  distinction  between  them.  But  in  astronomy,  where 
we  have  to  consider  bodies  in  the  heavens,  the  case  is  very 
different. 

Suppose  we  found  the  weight  of  a  ham  here  on  the  earth,  as 
determined  by  a  spring  balance,  to  be  24  pounds.  Suppose  we 
could  then  fly  up  to  the  moon  with  the  ham.  The  moon  has 
so  much  less  matter  than  the  earth  that  it  attracts  a  body  at 
its  surface  with  only  about  one  sixth  the  force  that  .the  earth 
attracts  the  body  at  its  surface.  Therefore  the  ham  on  the 
surface  of  the  moon  would  weigh  only  4  pounds  in  the  bal- 
ance. But  there  would  be  just  as  much  ham  there  as  there 
was  on  the  earth,  as  one  would  find  out  on  trying  to  eat  it. 
Therefore  the  mass  of  the  ham  would  be  the  same  as  before, 
although  the  weight  was  so  much  less. 


GRAVITATION  85 

If  instead  of  using  a  spring  balance  we  used  ordinary 
weights,  we  should  find  the  same  weight  on  the  moon  as  on  the 
earth,  because  the  weights  would  be  lighter  in  the  same  pro- 
portion as  the  ham.  is.  Therefore,  to  get  the  real  weight,  we 
suppose  a  spring  balance  to  be  used. 

If  a  baseball  club  could  fly  to  the  moon  and  there  play  a 
game,  the  distinction  between  mass  and  weight  would  be  very 
evident.  The  mass  being  the  same  as  here,  the  pitcher  would 
not  be  able  to  throw  the  ball  any  faster  than  he  can  throw  it 
on  the  earth.  The  catcher  would  find  the  ball  striking  as  heavy 
a  blow  in  his  hands  as  it  does  here,  and  the  batsman  could  bat 
it  no  faster  than  he  does  here. 

As  the  ball  would  be  drawn  toward  the  moon  by  only  one 
sixth  the  force  that  the  earth  draws  it,  it  would  be  found  to 
be  as  light  as  a  rubber  ball.  It  would  stay  long  in  the  air 
when  batted,  and  home  runs  would  be  made  all  the  time. 

As  the  quantity  of  matter  in  a  body  is  always  the  same, 
while  the  weight  varies  according  to  the  attraction  of  other 
bodies,  astronomers  do  not  speak  of  the  weight  of  heavenly 
bodies,  but  only  of  their  mass.  All  bodies  having,  the  same 
mass  attract  equally  at  the  same  distance.  Hence  the  mass  of 
a  body  may  be  determined  by  the  attraction  between  it  and 
another  body  at  some  fixed  place.  If  we  could  cut  a  planet 
into  pieces  small  enough  to  be  brought  to  the  earth  and 
weighed,  we  could  determine  the  mass  of  the  planet  by  the 
weight  of  the  pieces.  As  we  cannot  do  this,  we  determine  the 
mass  by  the  attraction  which  it  exerts  on  a  satellite  or  on 
some  other  planet. 

5.  How  the  Attraction  of  the  Sun  keeps  the  Planets  in  their 
Orbits.  —  In  consequence  of  universal  gravitation  all  the  heav- 
enly bodies  attract  each  other.  The  sun  is  so  very  large  and 
massive  that  it  attracts  the  planets  much  more  strongly  than 
they  attract  each  other.  We  have  now  to  see  that  the  planets 
move  round  the  sun  according  to  the  same  laws  that  govern 
the  motion  of  a  ball  thrown  by  the  hand. 


86 


ASTRONOMY 


Suppose  the  ball  thrown  in  the  line  AB,  figure  48.  If  there 
were  no  gravitation,  it  would,  in  a  certain  time,  say  one  second, 
reach  the  point  B,  and  in  two  seconds  the  point  D.  In  conse- 
quence of  gravitation  it  describes  a  curve  ACE,  as  every  one 
knows  who  watches  the  motion  of  a  ball.  The  distance  BC,  or 
the  drop  of  the  ball  from  its  line  of  throw  in  one  second,  will 
be  16  feet,  no  matter  whether  thrown  slowly  or  rapidly.  This 
is  the  distance  which  a  body  dropped  from  the  hand  will  fall  in 
one  second.  In  two  seconds  the  drop  of  the  ball  will  be  from 
the  point  D  to  E.  This  is  four  times  as  far  as  the  drop  in  one 


F 10.  48.  —  Showing  the  curved  path  of  a  ball  thrown  in  the  air  and  falling 
to  the  earth  under  the  attraction  of  gravitation. 

second,  because  the  attraction  of  the  earth  is  constantly  acting 
on  the  ball,  and  increasing  its  velocity,  thus  making  it  fall 
farther  every  second  than  it  did  the  second  before.  The  law 
is  a  drop  of  3  x  16  feet  during  the  second  second,  of  5  x  16  feet 
during  the  third,  etc.  Adding  up  the  falls  in  each  second,  we 
see  that  the  total  drop  will  be  4  x  16  feet  in  2  seconds,  9  x  16 
feet  in  3  seconds,  and  so  on,  as  the  square  of  the  time. 

A  little  study  will  make  it  plain  that  the  faster  the  ball  is 
thrown,  the  less  the  bending  or  curvature  of  its  path,  because 
it  must  go  farther  before  it  will  get  the  same  drop.  Thus  the 
course  of  a  bullet  seems  almost  straight,  while  a  ball  thrown 


GRAVITATION  87 

by  a  little  child  describes  a  path  much  more  curved  than  one 

batted  by  a  baseball  player. 

Imagine  the  earth  to  be  a  body  thrown  like  a  ball,  and 
attracted  by  the  sun.  To  learn  how  it  will  move,  we  must  note 
that  although  the  mass  of  the  sun  is  333,000  times  that  of  the 
earth,  yet  it  is  so  far  away  that  its  attraction  is  only  about 
i6*50  that  of  the  earth  on  things  about  us.  Hence  a  ton  weight, 


Sun 


FIG.  40.  —  Showing  a  small  part  of  the  earth's  orbit  round  the  sun.  In 
consequence  of  the  sun's  attraction,  it  is  continually  falling  away  from 
the  line  of  its  motion,  AB  for  example.  Compare  this  figure  with  the 
preceding  one,  and  note  that  as  a  ball's  path  continually  curves  toward 
the  earth,  so  the  earth's  path  continually  curves  toward  the  sun. 

or  2240  pounds,  is  here  on  the  earth  attracted  by  the  sun  with 
a  force  of  little  more  than  one  pound. 

To  find  how  far  a  body  like  the  earth  would  fall  toward  the 
sun  in  one  second,  we  must  divide  the  distance,  16  feet,  or  192 
inches,  by  1650.  This  is  less  than  J-  of  an  inch.  Now  in  figure 
49  let  the  arc  be  a  piece  of  a  circle  around  the  sun.  Draw  the 
line  AB,  touching  the  circle,  and  let  the  earth  be  thrown  in 
the  direction  of  this  line  with  such  speed  that  the  curvature 


88  ASTRONOMY 

of  the  path  in  consequence  of  the  fall  of  the  earth  toward 
the  sun  shall  be  equal  to  the  curvature  of  the  circle.  Then, 
notwithstanding  that  the  earth  has  commenced  to  fall  toward 
the  sun,  when  it  reaches  (7,  it  has  kept  in  this  circle  and 
is  no  nearer  to  the  sun  than  it  was  at  A,  but  is  now  going  in 
a  slightly  different  direction.  In  the  same  way,  when  it  has 
described  another  arc,  it  has  got  no  nearer  the  sun  by  its  falling, 
but  has  only  kept  in  the  circle.  All  that  the  sun  has  done  by 
its  attraction  is  to  keep  the  earth  from  flying  off  from  it  al- 
together in  a  straight  line.  It  keeps  bending  the  path  of  the 
earth  from  a  straight  line  into  the  circle  round  the  sun.  Thus, 
instead  of  either  falling  into  the  sun  or  going  away  altogether, 
the  earth  revolves  round  arid  round  the  sun  forever. 

The  idea  we  have  now  to  grasp  is  that  the  earth  is  not  held 
by  anything,  but  is  flying  through  space,  turning  on  its  axis 
all  the  while,  with  us  upon  it,  as  a  ball  might  fly  through  the 
air  with  insects  on  it. 

One  who  knows  enough  of  geometry  to  be  able  to  make  the 
necessary  calculations  will  find  that  in  order  that  the  earth 
may  thus  describe  a  circle  round  the  sun,  the  speed  with 
which  it  is  to  be  thrown  must  be  about  18.6  miles  per  second. 
That  is,  at  the  distance  of  18.6  miles  along  the  line  AB  the  circle 
round  the  sun  will  be  \  of  an  inch  from  this  line. 

But  the  earth  does  not  always  go  exactly  with  this  speed. 
We  must,  therefore,  show  what  happens  if  the  velocity  should 
be  a  little  less  than  that  we  have  supposed.  In  such  a  case, 
the  earth  or  other  planet  will  fall  a  little  nearer  the  sun  until  it 
gets  halfway  round.  But,  in  thus  falling  nearer  the  sun,  it 
will  have  acquired  a  greater  velocity,  and,  in  consequence  of 
this  increase  of  velocity,  it  will,  after  going  halfway  round, 
begin  to  recede  from  the  sun  until  it  gets  back  to  the  place  it 
started  from.  Thus  it  will  go  round  and  round  the  sun,  de- 
scribing an  orbit  a  little  nearer  the  sun  on  one  side  than  it  is 
on  the  other.  That  is,  the  sun  is  not  exactly  in  the  center  of 
the  orbit.  In  another  chapter  we  shall  see  that  the  orbit  is  an 
ellipse. 


GEAVITATION 


89 


6.  Centrifugal  Force,  —  Let  figure  50  represent  a  rapidly  re- 
volving wheel.  To  show  the  matter  clearly,  we  suppose  the 
rim  to  be  cut  into  eight  pieces  by  the  black  lines,  so  that  each 
piece  is  fastened  only  by  the  spoke.  Then  at  every  instant, 
in  virtue  of  the  first  law  of  motion,  the  parts  of  the  rim  will 
tend  to  fly  off  in  straight  lines  in  the  direction  in  which  they 
are  at  the  moment  moving.  This  direction  is  shown  by  the 
arrow  heads. 

But  they  are  kept  from  thus  fly- 
ing off  by  the  pull  of  the  spokes 
upon  them.  By  virtue  of  the  third 
law  of  motion  each  part  pulls  on 
the  spoke  with  the  same  force  that 
the  spoke  palls  on  it. 

This  pull  is  called  centrifugal 
force  because  its  direction  is  away 
from  the  center  on  every  side. 

The  swifter  the  motion,  the 
greater  the  centrifugal  force.  If 
the  speed  is  increased  without  limit,  the  force  will  become  so 
great  as  to  break  the  spokes.  Then,  each  piece  of  the  rim  will 
fly  away  in  the  straight  line  in  which  it  is  at  the  moment 
moving.  A  fly  wheel  regulating  the  motion  of  heavy  machin- 
ery is  sometimes  known  to  break  from  this  cause,  and  the 
pieces  flying  away  through  the  roof  of  the  building  and  the 
air  may  cause  great  damage. 

The  rotation  of  the  earth  on  its  axis  causes  a  slight  centrif- 
ugal force,  which  is  overcome  by  gravity.  One  of  its  effects 
is  to  make  all  bodies  on  the  earth's  surface  a  little  less  heavy 
than  they  would  be  if  the  earth  did  not  rotate. 

Another  effect  is  to  make  the  earth  and  planets  assume  the 
form  of  oblate  spheroids,  as  we  shall  explain,  for  the  earth,  in 
the  next  chapter. 


FIG.  50.  — Centrifugal  force. 


CHAPTER  VI 
THE  EARTH 

3.  Figure  and  Magnitude  of  the  Earth.  —  If  the  earth  did  not 
rotate,  the  attraction  of  every  particle  of  the  matter  composing 
it  upon  every  other  particle  would  tend  to  bring  it  into  the  form 
of  a  sphere.  But  the  rotation  of  the  earth  on  its  axis  gener- 
ates a  centrifugal  force  which  partially  counteracts  the  attrac- 
tion of  gravity  at  the  equator,  and  thus  makes  the  earth  bulge 
out  at  the  equator,  so  as  to  take  the  form  of  an  oblate  spheroid. 
In  this  form  of  spheroid  the  equator  is  a  circle,  and  the  axis 
or  diameter  through  the  pole,  called  the  polar  axis,  is  shorter 
than  that  through  the  equator. 

The  ratio  in  which  the  polar  axis  is  less  than  the  diameter 
at  the  equator  is  called  the  ellipticity  of  the  earth.  Its  amount 
is  about  -^ ;  perhaps  a  little  greater.  That  is,  if  we  repre- 
sent the  equatorial  diameter  by  the  number  300,  the  polar 
diameter  will  be  about  299. 

The  elevation  of  the  mountains  and  continents,  as  well  as 
the  depression  of  the  ocean  bottom,  make  the  real  figure  of  the 
solid  earth  slightly  irregular.  In  considering  the  general  fig- 
ure of  the  earth,  geodesists  conceive  of  it  as  if  the  earth  had 
been  put  into  a  turning  lathe  and  all  the  mountains  and  conti- 
nents planed  off  to  the  sea  level.  The  figure  thus  formed  by 
the  surface  of  ocean  and  planed-off  land,  is  called  the  geoid. 
The  figure  and  size  of  this  supposed  body  are  taken  as  the 
true  figure  and  size  of  the  earth,  on  which  the  continents 
and  mountains  are  regarded  as  excrescences. 

That  portion  of  the  matter  composing  the  earth  which  is 
near  its  surface  is  called  the  earth's  crust. 

90 


THE  EARTH 


91 


The  diameters  of  the  geoid  are :  — 

Equatorial  diameter 7926.5  miles. 

Polar  diameter      .        .        .        .        .        .        7899.5  miles. 

Thus  the  diameter  of  the  earth  is  about  27  miles  less  through 
the  poles  than  through  the  equator. 

The  surface  of  the  geoid  thus  denned  is  everywhere  at  right 
angles  to  the  line  of  gravity,  which  is  fixed  by  the  direction  of 
a  plumb  line.  Looking  at  figure  51,  we  see  that  this  line  PB 
does  not  point  exactly  at  the  center  of  the  earth,  except  at  the 
equator  E  and  the  poles.  The  difference  between  the  direction 
of  the  plumb  line  PB  and  the  line  PC  drawn  to  the  earth's 
center,  that  is,  the  angle  BPC,  is  called  the  angle  of  the  vertical. 

Its  greatest  amount  is  nearly  one-fifth  of  a  degree. 

2.  Latitude  and  Longitude. — By  the  astronomical  latitude  of  a 
point  on  the  earth's  surface  is  meant  the  angle  which  the  plumb 
line  at  that  point  makes  with  the  plane  of  the  equator.  In 
figure  51  the  latitude  of  the  point  P  is  the  angle  EBP.  It  is 
so  called  because  it  is  determined  by  astronomical  observa- 
tions. 


Pole 


Plane  of  Equator 

FIG.  51.  — Showing  the  difference  between  geographic 
and  geometric  latitude. 

The  geographical  latitude  of  a  place  is  the  same  as  its  astro- 
nomical latitude,  except  that  certain  small  deviations  in  the 
direction  of  the  plumb  line  are  allowed  for. 


92  ASTRONOMY 

The  geocentric  latitude  is  the  angle  which  the  line  from  the 
center  of  the  earth  to  the  place  makes  with  the  plane  of  the 
equator.  In  figure  51,  the  geocentric  latitude  of  the  point  P 
is  the  angle  ECP.  These  two  latitudes  differ  by  the  angle  of 
the  vertical. 

The  geocentric  latitude  of  a  place  cannot  be  directly  deter- 
mined, because  we  cannot  see  the  center  of  the  earth  nor  deter- 
mine its  exact  direction  by  observation.  But  we  can  always 
determine  the  direction  of  the  plumb  line  with  suitable  instru- 
ments. Hence  on  maps  and  for  ordinary  purposes  the  astro- 
nomical or  geographic  latitude  is  always  made  use  of. 

3.  Length  of  a  Degree.  — When  an  observer  stands  on  the  equa- 
tor, say  at  the  point  E,  figure  52,  the  plane  of  his  horizon  is  at 


f>/ane  of  the  Equator 
FIG.  62. 

right  angles  to  the  plane  of  the  equator.  If  he  travels  north, 
we  say  he  has  traveled  one  degree  when  his  horizon  has  changed 
its  position  by  one  degree  on  the  celestial  sphere.  The  further 
north  he  goes,  the  slower  his  horizon  will  turn  as  he  travels, 
and  consequently  the  further  he  must  go  in  order  that  the 
change  may  be  one  degree.  Hence  :  — 

The  degrees  of  latitude  are  shortest  at  the  equator,  and  continually 
grow  longer  as  we  approach  the  poles. 


THE  EARTH  93 

They  are  about  68.8  miles  in  length  at  the  equator  and  69.3 
miles  at  the  poles. 

At  the  equator  one  degree  of  longitude  is  a  little  more  than 
69  miles.  Owing  to  the  convergence  of  the  meridians  toward 
the  poles,  it  continually  shortens  as  we  approach  the  poles, 
where  it  becomes  nothing,  because  all  the  meridians  there 
meet. 

One  sixtieth  of  a  degree  —  that  is,  a  minute  of  arc  —  on  the 
earth's  surface  is  called  a  nautical  mile}  because  it  is  the  mile 
used  by  sailors.  The  latter  use  it  in  preference  to  our  land 
mile,  which  we  call  a  statute  mile,  because  they  determine  their 
positions  by  astronomical  observation  in  degrees  and  minutes, 
and  they  find  it  easy  to  take  one  minute  of  arc  on  the  earth's 
surface  as  a  mile.  This  is  nearly  a  mile  and  a  sixth. 

In  ordinary  cases  navigators  make  no  distinction  between 
the  lengths  of  a  degree  of  latitude  at  different  distances  from 
the  equator.  But  when  we  want  to  speak  of  the  length  of  a 
nautical  mile  with  exactness,  we  commonly  take  it  to  mean  a 
minute  of  longitude  at  the  equator. 

4.  How  the  Earth  is  Measured.  —  Owing  to  the  obstructions 
on  the  earth's  surface,  and  the  impossibility  of  fixing  points 
on  the  ocean,  we  cannot  measure  the  distance  round  the  earth 
as  we  would  measure  that  round  a  field  by  a  tape  line.  The 
determination  of  the  magnitude  and  figure  of  the  earth  must 
therefore  be  made  by  special  methods,  in  which  astronomical 
observation  and  measurement  of  distances  on  the  earth  are 
combined.  The  operation  of  measuring  large  portions  of  the 
earth's  surface  with  great  exactness  is  called  geodesy. 

Geodesy  requires  two  operations.  One  of  these  consists  in 
determining  the  exact  distance  between  two  points  on  the  earth 
in  meters,  yards,  or  miles.  This  is  done  by  a  process  called 
triangulation.  The  other  operation  consists  in  finding  out  by 
astronomical  observation  what  fraction  of  the  distance  round 
the  earth,  or  how  many  degrees  on  its  surface,  is  included  be- 
tween two  points  whose  distance  is  measured  by  triangulation. 


94 


ASTRONOMY 


Triangulation. —  The  principle  on  which  triangulation  is 
effected  is  this:  The  length  of  a  line,  AB,  figure  53,  is 
measured  as  exactly  as  possible  on  some  nearly  level  plane. 

A  line  thus  measured  for  geodetic  purposes  is  called  a  base 
line.  The  direction  of  the  base  line,  or  the  angle  which  it 

makes  with  the  meridian,  is  deter- 
mined by  astronomical  observation. 

A  distant  high  point  P  is  then 
chosen,  on  a  hill  or  mountain,  which 
can  be  seen  from  both  ends  of  the 
base  line.  The  angles  PAB  and 
PBA  are  measured  as  exactly  as  pos- 
sible with  a  theodolite.  Then,  by 
trigonometry,  the  sides  of  the  tri- 
angle BP  and  AP  can  be  exactly 
computed.  If  there  are  other  distant 
points,  like  Q,  which  are  visible  from 
the  two  ends  of  the  base  line,  tri- 
angles to  them  are  determined  in  the 
same  way,  and  their  distance  from 
each  other  computed. 

Any  of  the  sides  AP,  BP,  AQ,  BQ, 
or  PQ  can  then  be  used  as  a  new 
base  line,  and  the  positions  of  other 
distant  points  like  R  determined  by 
sighting  on  them.  By  sights  from 
these  points  other  yet  more  distant 
points  can  be  determined,  and  so  on  all  the  way  across  a  con- 
tinent if  necessary.  The  more  mountainous  a  region  is,  the 
easier  it  is  to  make  a  triangulation,  because  longer  sights  from 
one  mountain  top  to  another  can  be  taken  than  on  a  plain. 

Triangulation  on  a  large  scale  is  carried  on  by  the  United 
States  Coast  and  Geodetic  Survey.  The  latter  has  made  meas- 
ures across  the  American  continent  from  the  Atlantic  to  the 
Pacific  Coast.  Long  networks  of  triangles  have  also  been 
measured  in  various  parts  of  Europe,  Asia,  and  Africa. 


FIG.  53.  —  Example  of  a  tri- 
angulation. 


THE  EARTH  96 

If  the  earth  were  a  perfect  sphere,  the  determination  of  its 
magnitude  by  triangulation  and  astronomical  observation  com- 
bined, would  be  a  simple  problem.  Suppose  we  should  meas- 
ure a  north  and  south  arc  500  miles  in  length.  By  astronomical 
observation  we  find  the  difference  of  latitude  between  its  two 
ends  to  be  7°  12'.  Since  360°  reach  round  the  earth,  we  could 
state  the  proportion :  — 

7°  12' :  500  ::  360°  :  circumference  of  earth. 

This  would  give  25,000  miles  as  the  circumference.  We  might 
also  get  the  length  of  one  degree  by  dividing  500  by  7.2 ;  then  the 
circumference,  by  multiplying  the  quotient  by  360.  Dividing 
the  circumference  by  3.1416  would  give  us  the  earth's  diameter. 
This  shows  only  the  principle  by  which  the  problem  is 
solved.  The  actual  work  is  a  great  deal  more  complicated  and 
occupies  the  time  of  many  men,  year  after  year.  The  compli- 
cations arise  not  only  from  the  ellipticity  of  the  earth,  but 
from  the  fact  that  wherever  we  go,  the  direction  of  the  plumb 
line  is  slightly  changed  by  the,  attraction  of  hills,  mountains, 
and  continents,  and  also  by  that  of  matter  of  different  densities 
under  the  earth's  surface.  Even  when  these  irregularities  are 
allowed  for,  it  is  found  that  the  figure  of  the  geoid  has  many 
irregularities  which  have  not  yet  been  well  determined. 

5.  How  Latitude  and  Longitude  are  Determined.  —  The  second 
operation  of  geodesy  which  we  have  described  requires  the 
determination  of  the  exact  latitude  and  longitude  of  places  on 
the  earth's  surface.  This  determination  is  necessary,  not  only 
for  the  purposes  of  geodesy,  but  in  order  that  we  may  make 
exact  maps  of  counties  and  states,  lay  down  on  them  the  posi- 
tion of  cities,  and  find  the  distance  from  one  point  to  another. 
When  once  the  size  and  figure  of  the  earth  are  known,  it  is 
simpler  to  find  these  positions  and  distances  by  astronomical 
observation  than  it  is  to  measure  them  by  triangulation. 

Latitude.  —  To  determine  the  latitude  of  a  place,  the  astron- 
omer determines  the  exact  point  in  the  celestial  sphere  which 


96  ASTRONOMY 

corresponds  to  his  zenith.  He  might  do  this  in  a  rough  way  by 
sighting  upward  on  a  plumb  line,  and  noticing  what  stars 
were  near  his  zenith,  but  he  could  not  get  any  exact  result  by 
such  a  process  as  this.  The  principle  of  the  method  now  com- 
monly employed  is  this :  — 

Imagine  a  telescope  pointed  nearly  at  the  zenith,  and  a  spirit 
level  like  that  used  by  masons  and  architects,  only  much  more 
sensitive,  to  be  attached  to  it.  Fancy  the  telescope  to  be 
fastened  to  a  vertical  axis  which  turns  on  a  pivot  at  the  bot- 
tom. Adjust  this  axis  so  that  as  we  turn  the  telescope  round, 
the  level  shall  always  read  the  same.  Then  we  know  that  the 
line  of  sight  of  this  telescope  will  describe  a  circle  on  the 
celestial  sphere  with  the  exact  zenith  in  its  center.  The 
astronomer  finds  a  pair  of  stars  at  the  north  and  south  points 
of  this  circle,  of  which  he  knows  the  declinations.  Half  the 
sum  of  these  declinations  is  the  declination  of  the  zenith. 
This  is  equal  to  the  latitude  of  the  place,  as  will  be  seen  by 
§§11  and  12  of  Chapter  I. 

Yet  other  methods  may  be  u§ed.  We  have  explained  that 
the  latitude  of  a  place  is  equal  to  the  altitude  of  the  pole 
above  the  horizon.  It  is  also  equal  to  the  angle  between  the 
zenith  of  the  place  and  the  celestial  equator.  The  astronom- 
ical observer  can  determine  these  angles  with  great  precision, 
by  specially  constructed  instruments,  and  can  thus  obtain  his 
latitude  without  knowing  the  declinations  of  any  stars. 

Longitude. — We  have  already  shown  that  the  difference 
of  longitude  corresponds  to  the  difference  in  the  local  time 
at  two  places,  15°  of  longitude  always  corresponding  to  one 
hour's  difference  of  time.  We  may  also  define  the  differ- 
ence of  time  as  equal  to  the  time  which  it  takes  noon  to  travel 
from  one  place  to  the  other.  To  show  how  these  principles 
are  applied,  suppose  that  an  observer  at  New  York  telegraphs 
to  San  Francisco  the  exact  moment  at  which  the  sun  crosses 
his  meridian.  Then  when  the  sun  gets  to  San  Francisco,  an 
observer  there  telegraphs  to  New  York  the  moment  the  sun 
is  passing  the  meridian  of  San  Francisco.  The  elapsed  time 


THE  EARTH  97 

between  the  two  signals  would  be  the  time  required  by  noon 
to  travel  from  one  city  to  the  other.  Multiplying  the  hours, 
minutes,  and  seconds  by  15  would  then  give  us  the  degrees, 
minutes,  and  seconds  of  difference  of  longitude  between  the 
two  cities. 

The  same  result  is  found  by  each  observer  telegraphing  the 
other  at  a  given  moment  the  exact  sidereal  time  at  his  place.  He 
finds  the  time  by  noting  the  time  of  transit  of  stars  over  his  me- 
ridian with  a  transit  instrument  and  sidereal  clock.  To  make 
the  determination  with  the  greatest  exactness,  the  clock  is  so 
arranged  that  its  pendulum  shall  make  a  telegraphic  signal 
which  is  heard  at  the  distant  station  as  well  as  recorded  at  the 
station  where  the  clock  is.  Then  the  other  observer  sends  a 
signal  back  from  his  clock,  so  that  there  are  really  two  records 
of  the  same  difference  of  time  at  the  two  stations.  Of  these 
differences  one  will  be  a  little  too  great  and  the  other  a  little 
too  small  in  consequence  of  the  time  it  takes  electricity  to 
travel  from  one  station  to  the  other.  The  mean  of  the  two 
will  be  the  correct  difference  of  longitude  in  time. 

A  longitude  thus  determined  is  called  a  telegraphic  longitude. 
Difference  of  longitude  between  places  can  be  determined  by 
skillful  observers  in  this  way  with  an  error  of  only  a  few  hun- 
dredths  of  a  second  of  time,  or  a  few  yards  of  distance  on  the 
earth.  If  you  should  place  two  transit  instruments  three  or 
four  hundred  yards  east  or  west  of  each  other,  skillful  observers 
would  have  no  trouble  in  determining  their  distance  apart 
within  a  few  yards,  by  astronomical  observations  on  the  stars, 
combined  with  electric  signals  in  the  way  described. 

6.  Density  of  the  Earth,  Gravity,  etc.  —  By  the  density  of 
the  earth  is  meant  the  average  specific  gravity  of  the  material 
composing  it,  or  the  average  weight  of  a  cubic  foot  of  the 
earth's  matter  compared  with  that  of  a  cubic  foot  of  water. 
As  the  earth  is  composed  of  many  different  materials,  the 
specific  gravity  of  various  portions  of  it  is  very  different. 
What  we  want  is  the  mean  density  of  the  whole  earth. 
NEWCOMB'S  ASTRON.  —  7 


98  ASTRONOMY 

We  can  find  out  what  materials  compose  the  interior  of  the 
earth  only  by  digging  mines  so  as  to  get  at  them.  But  we 
cannot  dig  to  any  great  depth ;  only  in  the  rarest  cases  can 
we  go  three  or  four  thousand  feet  below  the  surface;  hence 
we  know  nothing  of  the  materials  that  compose  the  great  bulk 
of  the  interior  of  the  earth.  But  we  can  determine  the  mean 
density  of  these  materials  by  measuring  the  attraction  of  bodies 
whose  mass  is  known. 

Attraction  of  a  Sphere.  —  Imagine  a  sphere  of  lead  a  yard  in 
diameter.  Since,  by  the  law  of  gravitation,  every  particle  of 
matter  attracts  every  other  particle,  it  follows  that  this  sphere 
of  lead  must  attract  small  bodies  near  it.  The  attraction  is 
indeed  very  minute ;  it  can  be  made  sensible  only  by  exceed- 
ingly delicate  instruments.  But  in  recent  times  methods  have 
been  contrived  by  which  this  very  small  force  can  be  measured 
with  great  exactness.  We  have  to  see  how,  from  the  attrac- 
tion of  the  sphere  of  lead,  we  can  determine  the  mean  density 
of  the  earth. 


Fio.  54.  —  Attraction  of  two  spheres  of  different  sizes. 

Consider  two  spheres  of  matter,  A  and  B,  figure  54,  of  the 
same  density,  each  attracting  a  particle  P  at  its  surface.  Let 
A  be  twice  the  diameter  of  B.  It  is  found  by  mathematical 
processes  that  each  sphere  attracts  an  external  body  as  if  the 
whole  matter  of  the  sphere  were  concentrated  in  its  center. 

Sphere  A  being  twice  the  diameter  of  B,  has  eight  times  its 
mass.  Attraction  varying  directly  as  the  mass,  it  will  exert 
eight  times  the  attraction  of  B  at  the  same  distance 


THE  EARTH  99 

Attraction  also  varies  inversely  as  the  square  of  the  dis- 
tance, and  P  is  twice  as  far  from  the  center  of  A  as  from  that 
of  B.  Hence  the  same  matter  at  the  center  of  A  will  attract 
P  one  fourth  as  much  as  if  it  were  at  the  center  of  B. 

There  being  eight  times  as  much  matter  in  A,  its  actual 
attraction  on  P  will  be  double  that  of  B.  That  is :  — 

Spheres  of  equal  density  attract  bodies  at  their  surfaces  with  a 
force  which  varies  directly  as  their  diameter. 

Hence,  if  we  find  the  attraction  of  a  sphere  of  lead,  and  mul- 
tiply it  by  the  number  of  times  the  diameter  of  the  earth 
exceeds  that  of  the  sphere,  we  shall  have  the  attraction  of  a 
ball  of  lead  the  size  of  the  earth.  Comparing  this  with  the 
attraction  of  the  earth,  we  shall  have  the  ratio  between  the 
density  of  the  earth  and  that  of  lead.  It  is  thus  found 
that :  — 

Mean  density  of  the  earth  =  5£  times  that  of  water. 

This  is  much  greater  than  the  density  of  the  materials  com- 
posing the  earth's  crust,  and  results  from  the  enormous  force 
with  which  the  interior  of  our  globe  is  compressed  by  the 
weight  of  the  matter  around  it. 

7.  Condition  of  the  Earth's  Interior. — It  is  a  very  curious 
fact  that  when  a  mine  is  sunk  in  the  earth  the  temperature  is 
found  to  increase  with  the  depth.  The  rate  of  increase  is 
different  in  different  places,  but  is  commonly  not  far  from 
1°  Fahr.  in  50  feet.  At  the  depth  of  3000  feet  the  temperature 
would  therefore,  generally,  be  60°  above  the  mean  at  the  sur- 
face. This  temperature  is  so  high  that  miners  cannot  live  and 
work  at  the  bottom  of  a  deep  mine  except  by  having  cool  air 
pumped  down  to  them. 

There  is  every  reason  to  believe  that  the  increase  continues 
to  a  great  depth  at  the  same  rate,  so  that  a  few  miles  under- 
ground the  whole  earth  must  be  red-hot.  At  a  still  greater 
depth,  the  temperature  is  probably  sufficient  to  melt  all  the 
materials  of  which  the  earth  is  composed.  This  fact,  taken  in 


100  ASTRONOMY 

connection  with  the  phenomena  of  volcanoes,  has  led  to  the 
view  that  the  earth  is  really  a  mass  of  melted  matter,  with  a 
hard  crust  a  few  miles  thick,  on  which  we  live.  But  it  is 
found  that  the  earth  does  not  yield  to  the  attractive  forces 
of  the  sun  and  moon  as  it  would  if  it  were  liquid.  Hence,  the 
view  now  generally  accepted  is  that  the  materials  in  the  inte- 
rior are  kept  solid  by  the  enormous  pressure  to  which  the 
whole  interior  of  the  earth  is  subjected  by  the  mutual  gravita- 
tion of  its  parts.  How  great  this  pressure  is  can  be  con- 
ceived when  we  reflect  that  every  square  foot  a  hundred  miles 
below  the  surface  will  be  pressed  by  the  weight  of  a  column 
of  earth  a  foot  square  and  a  hundred  miles  high.  This  weight 
would  be  not  far  from  40,000  tons.  Such  a  pressure  would 
crush  any  substance  at  the  earth's  surface.  The  reason  the 
substance  inside  the  earth  is  not  crushed  is  that  it  is  pressed 
equally  on  all  sides  so  that  it  is  merely  condensed  into  a 
smaller  space  and  made  solid. 

8.  The  Atmosphere.  —  The  atmosphere  is  densest  at  the  sur- 
face of  the  earth,  and  grows  rarer  as  we  ascend  in  it.  This  is 
due  to  the  fact  that  every  part  of  it  is  pressed  by  the  weight 
of  the  whole  mass  of  air  above  it.  At  the  height  of  three 
miles  the  air  becomes  so  rare  that  most  people  find  a  difficulty 
in  breathing  at  such  a  height,  and  the  difficulty,  of  course, 
increases  with  the  height. 

The  temperature  of  the  air  continually  diminishes  as  we 
ascend.  Even  on  a  summer  day  it  is  generally  freezing  cold  at 
the  height  of  a  few  miles.  Hail  is  due  to  the  freezing  of  rain- 
drops in  the  cooler  region  of  the  air. 

Air  is  not  perfectly  transparent,  although  it  seems  to  be  so 
when  we  look  through  only  short  distances.  We  all  know 
that  when  we  look  at  objects  at  a  great  distance  they  have  a 
blurred,  hazy  aspect,  due  to  the  imperfect  transparency  of  the 
air  through  which  the  light  comes.  The  light  from  the 
heavenly  bodies,  as  it  passes  through  the  air,  is  diminished 
before  it  reaches  our  eyes  through  part  of  it  being  absorbed 


EARTH  /   ;  /.   -     ;  »•,•,  ~,  J.OJ; 

by  the  air  as  it  passes.  The  loss  is  smallest  at  the  zenith,  and 
increases  near  the  horizon,  because  the  rays  of  light  have  to 
pass  through  a  greater  distance  in  the  air  when  the  body  from 
which  they  come  is  near  the  horizon. 

The  blue  rays  are  more  absorbed  than  the  red  rays ;  hence 
a  larger  proportion  of  red  light  than  of  blue  light  reaches  our 
eyes  from  the  heavenly  bodies,  and  the  latter  look  more  or  Jess 
red  when  near  the  horizon.  This  is  why  the  sun  and  moon 
have  a  reddish  tinge  when  rising  or  setting. 

The  air  also  reflects  a  small  part  of  the  light  which  passes 
through  it ;  were  it  not  for  this,  the  sky  would  be  as  dark  by 
day  as  by  night,  and  we  should  see  the  stars  all  day.  There 
would  be  no  twilight,  because  darkness  would  come  on  as  soon 
as  the  sun  had  set.  Twilight  is  caused  by  the  reflection  of  the 
sunlight  from  the  upper  part  of  the  air  after  the  sun  has  set 
to  us. 

Twilight  ends  when  the  sun  is  about  18°  below  the  hori/on. 
This  shows  that  the  air  reflects  no  sunlight  at  a  height  greater 
than  45  miles.  This  is,  therefore,  commonly  taken  as  the 
limit  of  the  earth's  atmosphere.  But  the  phenomena  of  shoot- 
ing stars,  of  which  we  shall  speak  in  a  subsequent  chapter, 
show  that  there  is  really  some  kind  of  an  atmosphere  at  a 
height  of  nearly  100  miles.  But  we  do  not  certainly  know 
what  this  atmosphere  is. 

9.  The  Zodiacal  Light.  —  If  we  look  at  the  western  sky  on  a 
clear  evening  of  winter  or  spring  just  after  the  end  of  twilight, 
we  shall  see  a  very  faint,  soft  column  of  light  extending  along 
the  region  of  the  ecliptic,  and  gradually  fading  away  as  we  look 
farther  from  the  horizon.  The  same  appearance  may  be  seen 
in  the  eastern  horizon  before  daybreak,  in  the  summer  and 
autumn.  This  appearance  is  called  the  zodiacal  light,  because 
it  extends  along  the  region  of  the  zodiac.  In  our  latitudes  we 
cannot  see  it  in  the  evenings  of  summer  and  autumn,  because 
then  the  ecliptic  is  too  near  the  horizon,  and  the  light  is 
absorbed  by  the  thickness  of  the  air  through  which  it  has  to 


ASTRONOMY 


pass.      Within  the  tropics  it  may  be   seen  on  every  clear 
evening. 

There  is  something  mysterious  about  the  zodiacal  light,  but 
it  is  probably  caused  by  masses  of  very  tenuous  matter,  like 
fine  particles  of  dust,  which  circulate  around  the  sun  in  the 
whole  region  inside  the  orbit  of  Mars.  What  we  see  is  the 

sunlight  reflected 
from  these  very 
minute  particles. 

Connected  with 
the  zodiacal  light 
is  a  phenomenon 
called  the  Gegen- 
achein,  a  German 
word  signifying 
counter-glow.  It  is 
an  extremely  faint 
light  in  the  zo- 
diac, exactly  op- 
posite the  direc- 
tion of  the  sun.  Its 
faintness  is  such 
that  an  ordinary 
observer  would 
never  notice  it, 
nor  can  it  be  seen 
except  under  the 
most  favorable  cir- 


FIG.  55.  — The  zodiacal  light  as  seen  on  a  clear 
spring  evening. 


cumstances.  The  sky  must  be  very  clear ;  there  must  be  no 
moon  visible ;  the  observer  must  be  away  from  the  lights  of  a 
city ;  the  point  where  the  phenomenon  appears  must  not  be  in 
or  near  the  Milky  Way.  For  the  latter  reason  the  Gegenschein 
is  not  visible  in  June  or  July,  nor  in  December  or  January. 
Even  under  the  most  favorable  circumstances,  the  observer 
must  have  some  practice  in  seeing  a  faint  light  in  order  to  dis- 
tinguish it.  Its  cause  is  still  involved  in  mystery. 


CHAPTER  VII 


THE   SUN 


1.  Particulars  about  the  Sun.  —  The  sun  is  a  globe  whose 
diameter  is  more  than  a  hundred  times  the  diameter  of  the 
earth.  Hence  the  distance  round  it  is  more  than  a  hundred 
times  that  round  the  earth.  Because  the  volumes  of  globes 
vary  as  the  cubes  of 
their  diameters,  it  fol- 
lows that  the  volume 
of  the  sun  is  more 
than  a  million  times 
that  of  the  earth. 
More  exactly,  it  is 
1,297,000  times  that 
of  the  earth.  You 
understand,  without 
further  explanation, 
that  the  sun  looks 
small  because  of  its 
great  distance  of  93,- 
000,000  of  miles,  a 
distance  which  the 
swiftest  train  would 
not  run  in  a  hundred  FlG-  56.  — Showing  how  an  image  of  the  sun 

may  be  thrown  on  a  screen  with  a  spyglass, 
years. 

The  Sun's  Density,  Mass,  and  Gravity.  —  The  density  or  spe- 
cific gravity  of  the  matter  composing  the  sun  is  less  than  that 
of  the  matter  composing  the  earth.  The  mass  of  the  sun  is 

103 


104 


ASTRONOMY 


about  333,000  times  the  mass  of  the  earth,  instead  of  a  million 
times  and  more,  as  it  would  be  if  it  had  the  same  density. 

In  consequence  of  its  great  mass,  the  attraction  of  gravita- 
tion at  the  surface  of  the  sun  is  about  27  times  the  attraction 
of  the  earth  on  bodies  at  its  surface.  A  pound  of  matter  on 
the  earth  would  weigh  27  pounds  on  the  sun.  Under  an 
attraction  so  great,  a  man  of  ordinary  size  would  weigh  two 
or  three  tons,  and  would  therefore  be  crushed  to  death  by  his 
own  weight. 

The  Photosphere.  —  When  astronomers  speak  of  the  sun  they 
mean  the  whole  body  of  the  sun,  inside  as  well  as  outside. 
But  we  cannot  see  the  inside  of  the  sun ;  we  can  see  only  its 


'    FIG.  57.  —  Mottling  of  the  sun  as  photographed  by  Janssen. 

surface.  This  visible  surface  is  called  the  photosphere  or  light- 
sphere,  because  it  is  the  part  of  the  sun  which  sends  us  light 
and  heat. 


THE  SUN  105 

When  we  look  at  the  sun  with  a  good  telescope,  we  see  that 
the  photosphere  presents  a  mottled  appearance,  like  a  plate  of 
rice  soup.  The  grains  which  produce  this  mottling  are  hun- 
dreds of  miles  in  extent.  They  are  probably  caused  by  the 
matter  of  the  photosphere,  as  it  cools  off,  continually  falling 
back  into  the  still  hotter  interior  of  the  sun,  and  its  place 
being  taken  by  gaseous  matter  arising  from  inside,  as  we  shall 
next  describe. 

2.  Heat  of  the  Sun.  —  The  sun  shines  in  consequence  of  its 
very  high  temperature,  as  iron  shines  when  we  make  it  red- 
hot.  But  the  temperature  of  the  photosphere  is  much  higher 
than  that  of  red-hot  iron ;  higher  than  the  burning  coal  in  the 
hottest  furnace.  Possibly  the  temperature  in  the  most  power- 
ful electric  furnace  is  nearly  equal  to  that  of  the  photosphere. 
The  inside  of  the  sun  is  far  hotter  than  the  photosphere,  and 
there  is  reason  to  believe  that  it  gets  hotter  and  hotter  toward 
the  center. 

This  heat  is  so  intense  that,  subjected  to  it,  all  substances 
known  to  us  would  boil  away  like  water  over  a  fire  and  thus 
be  transformed  into  vapor.  Hence  it  is  believed  that  matter 
cannot  exist  in  a  solid  state  in  the  sun.  The  vapor  into  which 
the  substances  composing  the  sun  are  changed  by  the  fervent 
heat  is  so  compressed  by  the  enormous  gravitation  of  the  mass 
of  the  matter  around  it  that  it  is  forced  into  something  between 
a  gas  and  a  liquid.  The  intense  elastic  force  of  this  gaseous 
matter  causes  portions  of  it  to  be  continually  thrown  up  to  the 
sun's  surface,  or  to  the  region  of  the  photosphere.  There  it 
speedily  gets  colder  by  radiating  heat  into  space,  and  portions 
of  it  perhaps  condense  into  solids,  much  as  a  red-hot  crust  will 
form  on  the  surface  of  a  pot  of  melted  iron  taken  out  of  a 
furnace. 

It  would  not  be  correct  to  say  that  the  matter  of  the  sun  is 
burning,  because  things  are  said  to  burn  when  they  unite  with 
the  oxygen  of  the  air,  thus  producing  light  and  heat.  The  sun 
is  so  much  hotter  than  an  ordinary  fire  that  its  substance  could 


106  ASTRONOMY 

not  burn.  In  other  words,  it  differs  from  an  immense  fire  in 
being  so  much  hotter.  If  the  earth  should  fall  into  the  sun, 
everything  on  its  surface  would  be  melted  in  an  instant,  as  if 
a  small  ball  of  wax  fell  into  the  hottest  furnace. 

All  life  on  the  surface  of  the  earth  is  sustained  by  the  heat 
of  the  sun,  which  is  radiated  to  us  as  heat  from  a  fire  in  an 
open  fireplace  is  radiated  to  all  parts  of  a  room.  If  the  sun 
should  cease  to  give  us  heat,  the  air  and  the  whole  surface  of 
the  earth  would  slowly  cool  off.  In  a  few  days  it  would  be 
freezing  cold,  even  at  the  equator.  In  a  few  weeks  the  whole 
ocean  would  freeze  over,  and  the  soil  would  freeze  to  such  a 
depth  as  to  kill  every  plant.  Men  and  animals  might  be  able 
to  keep  alive  for  a  while  by  artificial  heat,  but  they  would 
soon  starve  in  consequence  of  not  having  anything  to  eat. 

3.  Spots  and  Rotation  of  the  Sun.  — When  the  sun  is  viewed 
through  a  telescope,  dark  looking  spots  are  frequently  seen  on 
his  surface.  These  spots  are  not  really  dark,  but  would  seem 
of  dazzling  brightness  against  the  sky  if  the  rest  of  the  sun 
were  not  there.  They  look  dark  only  in  contrast  to  the  intense 
brightness  of  the  photosphere. 

The  spots  are  of  various  sizes  and  shapes.  Occasionally  one 
appears  so  large  as  to  be  visible  to  the  naked  eye.  Commonly, 
however,  they  can  be  seen  only  with  the  telescope.  Sometimes 
a  number  of  small  ones  are  clustered  together,  forming  a  group 
of  spots. 

The  spots  are  extremely  irregular,  as  may  be  seen  from  the 
figures  which  we  give. 

The  central  part  of  a  spot  is  the  darkest.  It  is  called  the 
umbra,  or  nucleus.  Around  this  nucleus  is  a  border,  inter- 
mediate in  brightness  between  the  darkness  of  the  spot  and  the 
brilliancy  of  the  photosphere.  This  border  is  called  the 
penumbra. 

When  a  spot  is  carefully  examined  with  a  good  telescope  in 
a  steady  atmosphere,  it  is  found  to  be  striated,  looking  much 
like  the  bottom  of  a  thatched  roof,  the  separate  straws  bending 


THE  SUN  10? 

toward  the  interior  of  the  spot.     This  appearance  is  shown  in 
figures  59  and  60. 

Astronomers  are  not  agreed  as  to  the  nature  or  cause  of 
these  spots  on  the  sun,  though  they  have  been  studied  for 
nearly  three  centuries. 


FIG.  58.  —The  sun,  with  its  spots  and  prominences,  the  latter  being 
shown  by  the  spectroscope. 

The  Sun's  Rotation.  — When  the  spots  are  carefully  watched 
they  are  seen  to  change  their  position  from  day  to  day  by 
moving  slowly  across  the  photosphere  from  east  toward  west. 
In  this  way  it  is  found  that  the  sun,  like  the  earth,  rotates 
on  an  axis.  The  time  of  rotation  is  about  26  days.  The 
points  where  the  sun's  axis  of  rotation  intersect  its  surface  are 
called  the  poles  of  the  sun. 


108 


ASTRONOMY 


A  belt  around  the  sun,  90°  from  each  pole,  is  called  the 
sun's  equator.  It  is  found  by  watching  the  spots  that  they 
make  a  revolution  in  a  little  less  time  when  on  the  equator 
than  when  at  a  distance  from  it. 


FIG.  69.  — A  typical  solar  spot,  after  Langley,  showing  the  forms  which 
such  a  spot  often  presents. 


Periodicity  of  the  Spots.  — The  spots  are  much  more  numer- 
ous in  some  years  than  in  others.  In  years  when  spots  are 
scarce,  there  will  sometimes  be  none  visible  for  several  days, 
and  at  other  times  only  one  or  two  will  be  seen.  In  years 
when  spots  are  numerous,  quite  a  number  of  them,  and  some- 
times very  large  ones,  will  be  seen  nearly  all  the  time.  It  is 
found  by  the  records  of  the  years  when  spots  were  numerous 


TEE  SUN  109 

and  when  they  were  scarce,  that  there  is  a  period  of  about 
eleven  years,  during  one  half  of  which  there  are  few  spots, 
while  during  the  other  half  there  are  many. 


FIG.  60.  —A  solar  spot,  after  Secchi. 

S 

4.  Corona  and  Prominences.  —  When  the  sun  is  totally 
eclipsed  by  the  moon,  very  curious  and  beautiful  phenomena 
are  seen.  One  of  these  consist  of  red  patches  or  cloudlike 
forms  around  the  body  of  the  moon.  These  objects  are  called 
prominences  or  protuberances,  and  are  found  to  belong  to  the 
sun.  They  cannot  be  seen  with  a  telescope  when  there  is  no 
eclipse,  because  of  the  intense  light  of  the  sun  dazzling  our 
eyes.  This  is  why  we  see  them  only  when  the  light  of  the 
sun  is  cut  off  by  the  moon.  But  with  a  spectroscope  they  are 
visible  on  almost  any  clear  day.  This  instrument  shows  that 
they  are  composed  of  masses  of  gas,  mostly  hydrogen,  which 
are  from  time  to  time  shot  up  from  the  photosphere. 

Sometimes  they  have  the  form  of  immense  flames  blazing 
up  suddenly  to  a  height  of  many  thousand  miles  with  a 
velocity  of  more  than  100  miles  a  second.  In  such  flames 


110  ASTRONOMY 

everything  on  the  surface  of  the  earth  would  be  destroyed 
in  an  instant. 

Another  object  surrounding  the  sun  is  a  beautiful  effulgence 
called  the  sun's  corona.  It  cannot  be  seen,  even  with  a  spec- 
troscope, except  during  a  total  eclipse  of  the  sun.  It  will, 
therefore,  be  described  in  the  chapter  on  eclipses. 

5.  Source  and  Period  of  the  Sun's  Heat.  —  The  sun,  as  we  have 
already  explained,  is  merely  an  extremely  hot  body  radiating 
heat  to  us,  as  a  white-hot  globe  of  iron  would  radiate  heat  if 
hung  in  the  middle  of  a  room.  One  of  the  most  interesting 
and  important  questions  in  astronomy  is  why  the  sun  does  not 
cool  off  and  thus  gradually  cease  to  give  us  light  and  heat,  as 
the  iron  globe  would  do.  If,  as  is  generally  supposed,  the 
earth  and  sun  are  many  millions  of  years  old,  then  the  sun 
must  have  been  radiating  heat  during  this  immense  period. 
We  must,  therefore,  account  not  only  for  the  heat  the  sun  now 
gives  us,  but  for  the  heat  which  it  has  radiated  to  the  earth  in 
past  ages.  One  explanation  that  has  been  proposed  is  that  of 
meteors.  It  is  now  believed  that  there  are  great  numbers  of 
small  bodies  moving  round  the  sun  in  its  immediate  neighbor- 
hood, and  it  is  quite  likely  that  such  bodies  might,  from  time 
to  time,  fall  into  the  sun.  Each  body  thus  falling  would  gen- 
erate heat.  But  this  view  is  now  generally  given  up,  because 
it  seems  hardly  possible  that  meteors  in  sufficient  number  to 
generate  the  sun's  heat  could  be  falling  into  the  sun. 

The  view  now  commonly  held  is  that  the  heat  of  the  sun  is 
kept  up  by  the  constant  contraction  of  its  mass  through  the 
gravitation  of  its  particles  toward  the  center.  The  theory  of 
energy  teaches  us  that  heat  is  produced  when  a  body  falls 
toward  a  center  without  having  its  velocity  increased.  For 
example,  the  temperature  of  the  water  of  Niagara  Falls  must 
be  about  one-quarter  of  a  degree  higher  after  it  strikes  the 
bottom  than  it  is  before  it  goes  over  the  falls.  As  the  sun 
cools  off  it  must  grow  smaller,  so  that  its  outer  portions  fall 
toward  its  center.  In  this  falling  so  much  heat  is  acquired 


THE  SUN  111 

that,  if  the  sun  remains  gaseous,  it  will  continually  grow  hotter. 
It  may,  therefore,  continue  to  radiate  the  same  amount  of  heat 
every  year,  so  long  as  it  does  not  become  a  solid.  . 

If  this  view  be  correct,  a  time  must  come  when  the  sun  can 
contract  no  more.  Then  a  solid  crust  will  form  over  its  sur- 
face, this  crust  will  gradually  cool  off  by  the  heat  which  it 
radiates,  and  the  sun  will  gradually  grow  dark  and  cold.  But 
the  period  necessary  for  this  is  many  millions  of  years,  so  we 
need  not  trouble  ourselves  about  it. 

A  question  of  more  immediate  concern  is  whether  the  quan- 
tity of  heat  which  the  sun  gives  us  is  subject  to  variations. 
We  know  that  at  one  time,  probably  not  many  thousand  years 
ago,  the  whole  of  New  England  and  the  northern  states  was 
buried  all  the  year  round  in  snow  and  ice.  The  time  when  this 
was  the  case  is  called  the  Glacial  Epoch.  It  is  possible  that 
during  the  glacial  epoch  the  sun  gave  less  heat  than  it  does 
now.  If  so,  it  may  again  give  less  heat  at  some  future  time. 

There  is,  however,  no  evidence  of  any  change  in  the  tempera- 
ture of  the  earth  since  the  invention  of  the  thermometer.  The 
meteorological  observations  made  two  or  three  hundred  years 
ago  give  about  the  same  mean  temperature  that  they  do  in  our 
time.  But  the  earlier  observations  of  this  kind  are,  perhaps,  not 
very  reliable,  so  that  their  evidence  cannot  be  conclusive  against 
a  very  small  change  of  one  or  two  degrees.  But  the  observa- 
tions made  at  the  Greenwich  Observatory  from  1840  to  1890 
show  that  there  was  no"  perceptible  change  during  those  fifty 
years.  This  disproves  a  view  which  has  sometimes  been  main- 
tained, that  the  variations  in  the  solar  spots  produce  corre- 
sponding variations  in  the  temperature  of  the  earth.  It  also 
leads  us  to  believe  that  there  will  be  no  change  in  the  amount 
of  the  sun's  heat  for  many  years  to  come. 


CHAPTER  VIII 
THE  MOON  AND   ECLIPSES 

1.    Distance,  Size,  and  Aspect  of  the  Moon.  —  The  moon  is  a 

globe  like  the  earth.  Tt  looks  flat  to  the  eye  because  we  can- 
not see  its  roundness  without  a  telescope.  In  a  telescope  we 
can  see  it  to  be  round  like  a  globe. 

Distance.  —  The  moon  is  much  nearer  to  us  than  any  other 
of  the  heavenly  bodies.     Its  average  distance  is  a  little  less 


FIG.  61.  —  Showing  the  relative  size  of  the  earth  and  the  moon.  The 
diameter  of  the  moon  is  a  little  more  than  one  fourth  that  of  the 
earth. 

than  240,000  miles.  The  diameter  of  the  earth  being  nearly 
8000  miles,  the  distance  of  the  moon  is  about  30  times  the 
diameter  of  the  earth,  and  therefore  60  times  its  radius.  A 
railway  train  running  60  miles  an  hour  would  reach  the  moon 
in  five  or  six  months.  An  idea  of  the  relation  between  the 

112 


THE  MOON  AND  ECLIPSES  113 

distances  of  the  sun  and  moon  may  be  gained  by  remembering 
that  the  sun  is  nearly  400  times  as  far  as  the  moon. 

Size  and  Density.  —  The  diameter  of  the  moon  is  about 
2160  miles.  This  is  a  little  more  than  -£  of  the  diameter 
of  the  earth.  In  bulk  it  is  about  ^  that  of  the  earth.  But 
the  materials  which  compose  it  are  not  so  dense  as  those  of 
the  earth.  They  have  about  3  or  4  times  the  density  of  water. 
Thus  the  mass  of  the  moon  is  about  -  that  of  the  earth. 


FIG.  62.  —  Showing  the  size  and  distance  of  the  earth  and  moon  nearly 
in  their  true  proportions.  Their  distance  apart  is  about  30  diam- 
eters of  the  earth  and  more  than  110  that  of  the  moon. 

The  Moon's  Surface.  —  If  we  look  carefully  at  the  moon  near 
the  time  of  first  quarter  we  shall  see  little  irregularities  near  the 
left-hand  edge  of  the  bright  surface.  Through  a  telescope  this 
edge  looks  very  jagged.  This  is  because  the  surface  of  the 
moon  has  mountains  and  valleys  upon  it.  Sixty  or  seventy 
of  these  mountains  are  more  than  a  mile  high,  and  a  few  are 
four  miles  or  upward.  They  are  therefore  nearly  as  high  as 
the  highest  mountains  on  the  earth. 

But  the  shape  of  the  mountains  on  the  moon  is  very  differ- 
ent from  that  of  our  mountains  (see  figures  63  and  64).  Their 
tops  are  frequently  rounded  like  the  rim  of  a  saucer  or  shal- 
low plate,  the  inside  being  hollow,  and  black  like  the  bottom 
of  the  plate.  In  the  center  of  this  flat  region  there  is  very 
frequently  a  little  sharp  conical  peak. 

These  appearances  make  it  probable  that  long  ages  ago  these 
mountains  were  volcanoes.  There  is,  in  fact,  a  remarkable 
resemblance  between  these  lunar  hollows  and  the  craters  of 
volcanoes  like  Vesuvius.  A  hundred  years  ago  it  was  thought 
that  there  was  a  volcano  in  eruption  on  the  moon  ;  but  we 
now  know  that  this  was  a  mistake.  What  was  seen  was  only  a 
spot  of  unusual  brightness. 

NEWCOMB'S  ASTRON.  —  8 


114 


ASTRONOMY 


Some  parts  of  the  moon  are  much  darker  than  the  general 
surface.     It  is  said  that  Galileo  and  others  who  first  used   a 


FIG.  63.  —  The  moon,  photographed  by  Dr.  Henry  Draper. 


telescope  supposed  these  dark  portions  to  be  seas,  because  they 
looked  smoother  than  the  others.     Thus  Milton,  in  allusion  to 


THE  MOON  AND  ECLIPSES 


116 


Galileo,  who  was  a  native  of  Tuscany,  says  of  Satan's  shield 

that  it 

"Hung  on  his  shoulders  like  the  moon  -whose  orb 
Through  optic  glass  the  Tuscan  artist  views 
At  evening,  from  the  top  of  resole* 
Or  in  Valdarno,  to  descry  new  lands, 
Rivers  or  mountains  in  her  spotty  globe." 


FIG.  64.  —  Telescopic  view  of  a  region  on  the  moon. 

But  when  more  powerful  telescopes  were  made,  these  sup- 
posed seas  were  found  to  have  mountains  and  valleys  like  the 
rest  of  the  surface.  The  darkness  was  merely  the  result  of  a 
difference  of  shade  in  the  matter  forming  different  parts  of  the 
moon. 

Absence  of  Air  and  Water.  —  It  is  now  certain  that  the  moon 
has  neither  water  nor  air  in  any  quantity  sufficient  for  us  to 
detect  its  existence.  Consequently  there  is  no  weather  on  the 


116  ASTRONOMY 

moon  and,  so  far  as  we  have  yet  discovered,  nothing  ever 
happens  there,  except  that  the  surface  gets  warm  when  the 
sun  shines  on  it  and  cold  when  it  does  not. 

2.  The  Moon's  Revolution.  —  We  must  think  of  the  earth 
and  moon  as  two  companions,  revolving  round  the  sun  together, 
while,  at  the  same  time,  they  revolve  round  each  other.  The 
exact  truth  is  that  they  both  revolve  round  their  common 
center  of  gravity,  while  the  earth  goes  round  the  sun  in  the 
orbit  we  have  described.  Let  E  be  the  center  of  the  earth, 
M  that  of  the  moon,  and  C  their  common  center  of  gravity. 


FIG.  65.  — As  the  moon  moves  from  M  to  JV,  the  center  of  the  earth 
describes  the  small  arc  Ee  in  the  opposite  direction,  both  moving 
round  the  common  center  of  gravity  at  C. 

Then  EM  will  be  the  radius  vector  of  the  moon,  which 
means  the  line  from  the  center  of  the  earth  to  that  of  the 
moon.  We  must  now  conceive  that  this  radius  vector  turns 
round  on  C  as  on  a  pivot,  so  that,  while  the  moon  is  moving 
from  M  to  N,  the  earth  moves  from  E  to  e.  Thus  the  center 
of  the  earth  describes  the  small  dotted  circle,  while  at  the 
same  time  the  moon  describes  the  larger  circle  MN,  of  which 
only  an  arc  is  shown  in  the  diagram.  This  combined  motion 
arises  from  the  fact  that  the  moon  attracts  the  earth  as  much 
as  the  earth  does  the  moon. 


THE  MOON  AND  ECLIPSES 


117 


Sun 


The  common  center  of  gravity  C  is  really  inside  the  earth, 
about  one  fourth  of  the  way  from  its  circumference  to  its 
center.  Its  distance  from  the  earth's  center  is  therefore  so 
small  that  we  commonly  speak  of  the  moon  as  revolving  round 
the  earth,  without  reference  to  the  motion  of  the  earth  itself 
round  C. 

Sidereal  and  Synodic  Revolution.  —  Let  ABC  be*  an.  arc  of  the 
earth's  orbit  round  the  sun.  Let  us  start  with  the  earth  at  A, 
and  around  it  the 
orbit  of  the  moon, 
with  the  moon  at 
My  between  the  earth 
and  the  sun.  In  this 
position  the  moon  is 
said  to  be  in  conjunc- 
tion with  the  sun. 

While  the  earth 
is  moving  from  A 
to  By  the  moon 
makes  one  revolu- 
tion around  it,  and 
reaches  the  point  N 
such  that  the  line 
BN  is  parallel  to 
the  line  AM.  These 
lines  being  parallel, 
the  moon  has  made 
a  complete  revolu- 
tion, and  is  seen  in 


V 


-  66.  —  Showing  the  difference  between  the 
sidereal  and  synodic  periods  of  the  moon. 


the  same  real  direc- 

,    ,,         -, 
tion  at  N  as  she  was 

at  M.  This  revolution  of  the  moon  around,  the  earth  is  called 
a  sidereal  revolution  because,  when  it  is  completed,  the  moon 
has  returned  to  the  same  apparent  point  among  the  stars.  It 
takes  place  in  about  27  d.  8  h. 

Although  the  moon  has  actually  made  one  revolution  round 


118  ASTRONOMY 

the  earth  when  it  comes  to  N,  yet  she  will  not  be  in  conjunction 
with  the  sun  at  N,  but  will  have  to  move  through  an  arc  NP  to 
catch  up  to  where  the  sun  appears  to  be.  This  takes  it  more 
than  two  days  more.  Thus  the  time  between  the  moon's  con- 
junctions with  the  sun  is  on  the  average  29  d.  13  h.  This  period 
between  two  conjunctions  with  the  sun  is  called  a  synodic  revo- 
lution. 

3.  The  Moon's  Phases  and  Rotation.  —  The  moon  is  an  opaque 
body  which  shines  only  by  reflecting  the  light  of  the  sun.  That 
hemisphere  which  is  toward  the  sun  is  always  brightly  illumi- 
nated by  the  sun's  rays ;  the  other  is  in  darkness  so  that  we 
do  not  plainly  see  it. 

When  the  moon  is  in  conjunction  with  the  sun,  her  dark 
side  is  turned  toward  us,  and  we  cannot  see  her  at  all.  The 
almanacs  then  call  it  new  moon,  though  we  cannot  see  the 
moon. 

Two  or  three  days  later  she  has  moved  away  from  the  sun 
so  far  that  a  small  portion  of  her  illuminated  hemisphere  is 
visible.  The  form  which  she  then  shows,  and  with  which  we 
are  so  familiar,  is  called  a  crescent,  because  the  moon  is  then 
increasing. 

At  this  time,  if  we  look  carefully,  we  shall  see  the  entire 
round  disk  of  the  moon,  the  dark  part  having  a  very  faint  gray 
illumination.  This  is  caused  by  the  light  from  the  earth  being 
reflected  upon  the  moon.  The  earth  being  several  times  larger 
shines  much  more  brightly  upon  the  moon  than  the  moon  does 
upon  the  earth.  The  appearance  is  familiarly  called  "  the  old 
moon  in  the  new  moon's  arms." 

In  three  or  four  days  more  the  moon  has  got  to  the  posi- 
tion of  first  quarter.  One  half  the  illuminated  hemisphere  is 
now  visible  to  us  and  her  visible  disk  has  the  form  of  a  semi- 
circle. 

During  the  next  few  days  we  see  more  and  more  of  the 
illuminated  hemisphere,  and  the  moon  is  said  to  be  gibbous. 

When  the  moon  gets  opposite  the  sun  she  presents  the  same 


THE  MOON  AND  ECLIPSES 


119 


face  to  the  earth,  and  to  the  sun.     We  see  her  whole  illumi- 
nated hemisphere  and  call  it  full  moon. 

During  the  second  half  of  the  revolution  the  phases  recur  in 
the  reverse  order,  and  a  week  after  full  moon  she  has  got  round 
through  another  quarter  of  her  journey.  We  then  say  that  she 
is  in  her  third  quarter.  We  can  then  again  see  one  half  the 
illuminated  hemisphere. 


Direction  of  Sun> 


FIG.  67.  —  The  moon's  phases. 


In  7  or  8  days  more  she  is  again  in  conjunction  with  the  sun 
and  we  lose  sight  of  her. 

The  age  of  the  moon  is  the  time  elapsed  since  new  moon. 
When  we  first  see  her  as  a  thin  crescent  after  sunset,  she  is 
commonly  2  or  3  days  old.  At  first  quarter  she  is  7  or  8 ; 
at  full  moon  about  15 ;  at  last  quarter  22  days  old. 

The  best  time  to  see  the  moon  through  a  telescope  is  not 
when  she  is  full,  as  people  commonly  suppose,  but  when  she  is 
between  4  and  8  days  old. 


120  ASTRONOMY 

Form  of  the  Moon's  Orbit.  —  We  shall  explain  in  another 
chapter  that  the  planets  move  round  the  sun  in  ellipses  having 
very  nearly  the  form  of  a  circle.  If  the  earth  and  moon  were 
attracted  by  no  body  but  the  sun,  they  would  move  around 
each  other  in  ellipses,  as  the  planets  move  round  the  sun.  But 
the  sun  attracts  both,  and  thus  prevents  the  orbit  being  an 
exact  ellipse,  and  also  makes  it  change  its  form  slightly,  but 
continually.  The  result  is  that  the  orbit  is  much  like  a  mov- 
ing ellipse.  The  point  of  this  ellipse  where  the  moon  comes 
nearest  the  earth  is  called  the  perigee  ;  tlHit  where  she  is  far- 
thest is  called  the  apogee. 

The  positions  of  the  apogee  and  perigee  are  continually 
changing,  and  they  make  a  complete  revolution  round  the 
earth  in  about  nine  years. 

The  Moon's  Effect  on  the  Weather.  —  It  used  to  be  supposed 
that  the  moon  had  some  effect  on  the  weather,  and  that  changes 
of  weather  were  more  likely  to  occur  at  new  or  full  moon,  or 
at  one  of  the  quarters.  It  is  now  known  that  this  is  not  the 
case.  The  most  careful  observations  show  that  the  moon  has 
no  effect  at  all  on  the  weather. 

Rotation  of  the  Moon.  —  As  the  moon  revolves  around  the 
earth,  she  always  presents  nearly  the  same  face  toward  us. 
This  shows  that  she  turns  on  her  axis  in  the  same  time  that 
she  revolves  around  the  earth.  It  should  be  noticed  that  if 
the  moon  did  not  turn  on  her  axis  at  all,  then  as  she  went 
round  the  earth  we  should  see  her  from  various  directions,  and 
so  should  get  a  view  of  all  parts  of  her  surface. 

As  she  always  turns  the  same  face  toward  us,  it  follows  that 
we  can  never  see  the  other  side  of  the  moon.  But  there  are 
small  changes  in  the  speed  with  which  she  performs  her  revolu- 
tion round  the  earth,  while  her  rotation  on  her  axis  is  uniform. 
Hence  we  can  sometimes  see  a  little  farther  on  one  side  or  the 
other  of  her  body.  Such  an  appearance  is  called  libration. 
This  word  means  a  balancing,  and  is  applied  because,  to  our 
eyes,  the  moon  seems  to  have  a  slight  swing  back  and  forth  on 
her  axis,  as  a  balance  has  when  the  weights  in  the  pans  are  equal. 


THE  MOON  AND  ECLIPSES  121 

4  The  Tides.  —  In  consequence  of  its  gravitation,  the  earth 
attracts  the  moon  and  thus  keeps  her  in  her  orbit.  If  it  were 
not  for  this  attraction  the  moon  would  gradually  leave  the 
earth  altogether,  as  has  already  been  explained.  But,  by  the 
third  law  of  motion,  the  moon  attracts  the  earth  as  well  as 
the  earth  the  moon.  Hence  the  earth  is  being  continually 
drawn  toward  the  moon.  But  it  can  never  move  far  in  con- 
sequence of  this  drawing,  because  of  the  constantly  changing 
direction  in  which  the  moon  acts :  at  one  time  of  the  month 
the  attraction  is  in  "one  direction,  and  at  the  opposite  time  in 
the  other  direction. 


D 
FIG.  68.  —  Showing  how  the  moon  causes  the  tides. 

We  have  already  said  that  gravitation  is  less  the  greater  the 
distance.  Hence  the  portion  of  the  earth  near  the  moon  is 
attracted  more  strongly  than  the  portion  most  distant  from  it. 
The  result  is  that  the  attraction  of  the  moon  tends  to  draw  the 
earth  out  into  an  ellipsoidal  form.  The  earth  itself,  however, 
being  a  solid  body,  cannot  be  stretched  out  by  this  increased 
attraction.  But  the  water  of  the  ocean,  being  movable,  is 
stretched  out  a  little.  Thus  a  wave,  very  broad,  but  only  a 
few  feet  deep,  is  made  in  the  ocean,  and  follows  the  moon 
around  every  day.  There  is  also  a  similar  wave  on  the  oppo- 
site side  of  the  earth.  This  is  because  at  that  point  the  water 
is  attracted  less  than  the  average  of  the  solid  earth,  so  that  the 
moon  pulls  the  earth  away  from  the  water.  Thus  there  are 
two  waves  a  day  moving  round  the  earth. 

These  waves  are  called  tidal  ivaves.  The  rise  and  fall  of  the 
water  of  the  ocean  which  they  produce  are  called  tidies.  They 


122  ASTRONOMY 

strike  our  coast  and  make  the  water  rise  for  6  hours,  until 
the  top  of  the  wave  reaches  us.  It  is  then  called  high  tide. 
During  the  next  6  hours  the  tide  recedes.  At  its  lowest  it  is 
called  low  tide.  Six  hours  later  there  is  another  high  tide,  and 
so  on.  Thus  there  is  a  regular  rise  and  fall  of  the  water  twice 
every  day,  with  which  all  who  live  on  the  seacoast  are  familiar. 

In  consequence  of  the  continual  motion  of  the  moon  on  the 
celestial  sphere,  from  west  toward  east,  she  passes  the  meridian 
on  the  average  about  50  minutes  later  every  day  than  she  did 
the  day  before.  Hence,  the  tides  arrive  later  every  day  by 
this  average  amount. 

The  amount  of  the  rise  and  fall  is  very  different  in  different 
regions.  Out  in  the  ocean  it  is  generally  less  than  on  the 
coast,  commonly  only  2  or  3  feet.  As  the  tidal  wave  ap- 
proaches a  coast  the  resistance  of  the  latter  causes  the  water 
to  pile  itself  up  against  the  coast,  and  thus  rise  to  a  height  of 
6,  10,  or  20  feet,  or,  in  rare  cases,  much  more. 

Owing  to  the  islands  and  continents,  the  tidal  wave  is  not 
merely  one  wave  going  along  uniformly,  but  sometimes  there 
are  several  waves  in  different  parts  of  the  same  ocean.  When 
two  of  these  waves  happen  to  meet,  they  make  one  big  wave. 
If  there  happens  to  be  a  deep,  wide-mouthed  bay  where  they 
meet,  the  water  may  rise  to  a  very  great  height  in  consequence 
of  the  force  with  which  it  enters  the  bay.  This  is  the  case  in 
the  Bay  of  Fundy,  on  the  coast  of  Nova  Scotia  and  New 
Brunswick.  At  the  head  of  this  bay  the  tides  rise  70  or  80 
feet.  The  effect  is  here  most  extraordinary.  The  Basin  of 
Minas  is  quite  a  large  lake,  at  high  tide  being  12  miles  across 
and  40  miles  long.  But  at  low  tide  it  is  almost  empty. 

Spring  and  Neap  Tides.  —  The  attraction  of  the  sun  on  the 
earth  produces  a  tide  as  well  as  that  of  the  moon.  But  this 
tide  is  smaller  than  that  of  the  moon.  At  the  times  of  new 
and  full  moon,  the  sun  and  moon  unite  their  attraction  to  pro- 
duce tides.  Consequently  the  tides  are  higher  at  those  times 
than  at  others.  These  are  called  spring  tides. 

At  first  and  last  quarter  the  sun  and  moon  pull  against  each 


THE  MOON  AND  ECLIPSES  123 

other  on  the  tides.  Thus  the  sun.  diminishes  the  effect  of  the 
inoon,  and  the  tides  are  not  so  high.  They  are  then  called 
neap  tides. 

Another  effect  of  this  combined  action  of  the  sun  and  moon 
is  that  the  actual  intervals  between  the  high  tides  on  succes- 
sive days  sometimes  vary  considerably  from  the  average  inter- 
val. Sometimes  high  or  low  tide  occurs  at  nearly  the  same 
time  on  two  successive  days.  At  other  times  the  difference  of 
time  may  be  more  than  an  hour. 

5.  Eclipses  of  the  Moon.  —  All  opaque  bodies  cast  shadows 
when  the  sun  shines  on  them.  Hence  the  moon  and  the  earth 
cast  shadows.  Night  is  caused  by  our  being  in  the  shadow  of 
the  earth  when  our  hemisphere  is  turned  away  from  the  sun. 


.  69. 


Let  S,  figure  69,  be  the  sun,  E  the  earth,  and  M  the  moon. 
Draw  the  lines  ABH  and  CDH  meeting  at  H,  and  touching 
the  sun  and  earth.  You  will  then  see  that  between  these  two 
lines,  in  the  region  between  the  earth  and  H,  the  light  of  the 
sun  will  be  cut  off.  This  region  is  that  of  the  shadow  of  the 
earth.  The  shadow  has  the  shape  of  a  cone,  with  its  point  at 
H.  This  is  called  the  shadow  cone. 

Outside  the  shadow  is  a  region  PP  in  which  the  light  of  the 
sun  is  partly  but  not  wholly  cut  off.  An  observer  in  this 
region,  if  he  could  fly  up  to  a  great  distance  from  the  earth, 
would  see  the  latter  hide  a  greater  or  less  part  of  the  sun, 
according  to  his  nearness  to  the  surface  of  the  shadow  cone. 

The  region  PP  in  which  the  sunlight  is  partly,  but  not 
wholly,  cut  off,  is  called  the  penumbra. 


124  ASTRONOMY 

When  the  moon  is  entirely  in  the  shadow  of  the  earth,  the 
direct  light  of  the  sun  can  no  longer  reach  her,  so  she  looks 
dark.  We  then  say  that  there  is  an  eclipse  of  the  moon.  That 
is,  an  eclipse  of  the  moon  is  caused  by  the  moon  passing  through 
the  shadow  of  the  earth. 

It  is  very  interesting  to  watch  such  an  eclipse.  As  the  moon 
enters  the  shadow  we  see  a  small  part  of  one  edge  of  her  disk 
grow  dark  and  finally  disappear.  The  darkness  spreads  over 
the  disk  little  by  little  until  it  covers  the  whole  surface  of  the 
moon.  During  the  first  part  of  the  eclipse  we  cannot  see  the 
eclipsed  portion  of  the  moon  because  of  the  dazzling  effect  of 


FIG.  70.  —  Refraction  of  the  light  of  the  sun  into  the  earth's  shadow. 

the  bright  part.  But  when  the  bright  part  has  nearly  or  quite 
disappeared,  we  see  the  whole  disk  shining  with  a  dim,  reddish 
light.  This  is  because  the  light  of  the  sun  is  refracted  by  the 
earth's  atmosphere  as  we  have  explained  in  Chapter  IV,  and 
shown  by  the  above  figure.  Hence  the  rays  of  the  sun,  which 
pass  very  near  the  surface  of  the  earth,  are  so  refracted  by  the 
air  that  they  enter  the  shadow,  and  keep  it  from  being  perfectly 
dark.  To  an  observer  on  the  moon,  looking  at  the  earth  during 
an  eclipse,  the  sun  would  be  entirely  hidden  by  the  earth,  but 
the  latter  would  be  surrounded  by  a  thin  ring  of  this  refracted 
light,  of  a  reddish  tint.  This  tint  is  due  to  the  absorption  of 


THE  MOON  AND  ECLIPSES  125 

the  blue  rays  by  the  atmosphere,  and  hence  arises  from  the 
same  cause  that  makes  the  sun  look  red  when  on  the  horizon. 

Sometimes  only  a  part  of  the  moon  dips  into  the  shadow. 
The  eclipse  is  then  called  a  partial  edipse  of  the  moon. 

When  the  moon  is  altogether  immersed  in  the  earth's  shadow, 
the  eclipse  is  said  to  be  total. 

6.   The  Moon's  Orbit  and  Nodes.  —  You  may  now  ask  why  it 

is  that  there  is  not  an  eclipse  of  the  moon  at  every  full  moon, 
because  the  moon  is  then  always  opposite  the  sun.  The  reason 
is  that  the  shadow  of  the  earth  is  always  in  the  ecliptic,  while 
the  orbit  of  the  moon  around  the  earth  is  inclined  to  the  plane 
of  the  ecliptic,  as  shown  in  figure  71. 


FIG.  71. — Showing  the  inclination  of  the  moon's  orbit  to  the  plane  of 
the  ecliptic.  The  latter  is  represented  by  the  horizontal  surface  ;  the 
plane  of  the  moon's  orbit  by  the  inclined  circle.  The  latter  inter- 
sects the  ecliptic  at  the  points  M  and  JV,  which  are  called  nodes.  The 
line  joining  the  two  nodes  is  called  the  line  of  nodes. 

If  we  imagine  ourselves  standing  on  the  earth  at  E,  and 
mapping  out  the  moon's  course  among  the  stars,  as  we  have 
imagined  the  apparent  course  of  the  sun  to  be  mapped  out, 
the  two  courses  would  not  be  the  same,  but  would  intersect 
each  other  at  two  opposite  points.  When  the  moon  was  in  the 
half  A  of  her  orbit,  she  would  appear  north  of  the  plane  of  the 
ecliptic,  and  when  in  the  half  B,  south  of  it. 

There  are  two  opposite  points,  M  and  Nt  at  which  the  orbit 


126  ASTRONOMY 

intersects  the  ecliptic.  These  points  are  called  nodes.  The 
straight  line  joining  the  nodes  passes  through  the  center  of 
the  earth,  where  also  the  plane  of  the  moon's  orbit  intersects 
that  of  the  ecliptic.  It  is  called  the  line  of  the  nodes. 

Figure  72  shows  four  positions  of  the  earth's  shadow,  when 
it  happens  to  be  near  one  of  the  moon's  nodes.  As  the  moon 
moves  along  her  orbit  she  crosses  the  ecliptic,  and,  if  the 
shadow  happens  to  be  near  the  same  point,  may  enter  it  in 
whole  or  in  part,  according  to  the  distance  from  the  node. 


FIG.  72. — The  dotted  circles  show  different  positions  of  the  earth's 
shadow  near  the  moon's  node.  The  shadow  is  always  opposite  the 
sun  ;  but  we  cannot  really  see  it.  As  the  moon  passes  along  it  may 
enter  the  shadow  partially  or  entirely,  then  we  see  it  more  or  less 
eclipsed.  The  smaller  circles  represent  the  moon.  In  the  two  right 
hand  positions  the  moon  is  wholly  immersed  in  the  shadow ;  in  the 
left  hand  position  it  does  not  enter  the  shadow  at  all. 

The  inclination  of  the  moon's  orbit  to  the  ecliptic  is  about  5°. 
This  is  ten  times  the  apparent  diameter  of  the  moon. 

7.  Eclipses  of  the  Sun.  —  An  eclipse  of  the  sun  is  a  partial  or 
entire  hiding  of  the  sun  through  the  intervention  of  the  moon. 

Of  course  the  moon  casts  a  shadow  as  the  earth  does.  Figure 
73  shows  its  form.  We  draw  the  lines  SM  and  TN  from  the 
edge  of  the  sun,  meeting  in  the  point  H.  Here  the  shadow 
ends  in  a  sharp  point.  To  an  observer  in  the  shadow  the  sun 
will  be  completely  hidden  by  the  dark  body  of  the  moon.  The 
eclipse  is  then  said  to  be  total. 

Now  draw  lines  TM  and  SN9  touching  the  moon.  Between 
these  lines  and  the  shadow  is  the  penumbra.  An  observer  in 
this  region  will  see  the  sun  partly  hidden  by  the  moon.  This 
phenomen  is  called  a  partial  eclipse  of  the  sun. 


THE  MOON  AND  ECLIPSES  127 

The  average  distances  of  the  sun  and  moon  from  the  earth 
are  such  that  the  point  H  of  the  shadow  is  generally  very  near 
the  surface  of  the  earth.  When  the  moon  is  near  perigee  the 
shadow  does  not  quite  come  to  a  point  before  reaching  the 
earth.  There  will  then  be  a  small  region  of  the  earth's  surface 


FIG.  73.  —  The  moon's  shadow  and  penumbra.  This  figure  shows  the 
shape  of  the  dark  shadow  of  the  moon.  To  an  observer  inside 
the  shadow  the  moon  will  entirely  hide  the  sun.  An  observer  in 
the  penumbra  will  see  the  sun  partially  covered  by  the  moon.  An 
observer  at  the  point  of  the  shadow  will  see  the  moon  exactly  cover 
the  sun.  Sometimes  the  surface  of  the  earth  is  just  at  the  point, 
sometimes  inside  and  sometimes  outside,  according  to  the  varying 
distance  of  the  moon. 

on  which  the  eclipse  will  be  total.  As  the  moon  moves  in  its 
orbit  the  shadow  sweeps  along  a  path  on  the  earth's  surface. 
This  is  called  the  path  of  total  eclipse.  An  observer  cannot 
see  the  eclipse  as  total  unless  he  places  himself  somewhere 
along  this  path.  In  the  astronomical  ephemeris  maps  are  given 
showing  the  paths  of  all  the  total  eclipses  that  occur.  These 
are  of  various  breadths,  but  are  generally  less  than  a  hun- 
dred miles  wide. 

There  can  be  no  eclipse  unless,  at  the  time  of  new  moon,  the 
sun  is  near  one  of  the  moon's  nodes.  If  this  is  not  the  case 
the  moon  will  seem  to  pass  above  or  below  the  sun.  The 
various  kinds  of  eclipses  of  the  sun  can  be  best  understood  by 
studying  figure  74.  The  dark  body  of  the  moon  is  shown  in 
five  positions,  while  it  is  passing  the  sun,  as  .it  would  appear 


128  ASTRONOMY 

if  we  could  see  it,  which,  however,  we  cannot  do  except  as  we 
may  see  the  sun  eclipsed. 

In  position  A,  when  the  sun  has  not  reached  the  node,  the 
moon  passes  a  little  below  the  sun.  This  causes  a  partial 
eclipse,  the  appearance  of  which  the  figure  shows. 

In  the  position  B,  the  centers  of  the  sun  and  moon  coincide 
exactly  at  the  moment  of  conjunction.  The  eclipse  is  then 
said  to  be  central.  There  are  two  kinds  of  central  eclipses : 
total  and  annular. 


FIG.  74.  — This  figure  shows  how  at  new  moon  there  may  be  a  central, 
total  or  annular  eclipse  of  the  sun,  a  partial  eclipse,  or  no  eclipse  at 
all,  according  to  the  distance  of  the  sun  from  the  moon's  node.  In 
each  figure  the  round  white  circle  represents  the  sun  and  the  black 
circle  the  dark  invisible  body  of  the  moon  which  may  pass  over  it. 
In  the  position  B  the  sun  is  exactly  at  the  moon's  node,  so  that  the 
dark  body  of  the  moon  passes  centrally  over  the  sun.  At  A  the 
moon  passes  a  little  below  the  sun  and  at  C  a  little  above  it,  cover- 
ing the  greater  part.  When  the  sun  is  still  farther  from  the  node, 
as  at  Z),  the  moon  covers  only  a  small  portion  of  it.  When  the  sun 
is  still  farther,  as  at  E,  the  moon  passes  above  it  entirely  so  that 
there  is  no  eclipse.  When  the  moon  is  off  the  sun,  as  at  E,  we  can- 
not see  it  in  the  heavens  but  we  can  imagine  its  position  as  shown 
in  the  figure. 

If  the  apparent  diameter  of  the  moon  is  greater  than  that  of 
the  sun,  as  will  be  the  case  when  the  moon  is  near  perigee, 
the  sun  will  be  entirely  hidden  and  the  eclipse  will  be  total. 

If  the  apparent  diameter  of  the  moon  is  a  little  less  than 
that  of  the  sun,  which  will  be  the  case  when  the  moon  is  near 
apogee,  the  edge  of  the  sun's  disk  will  be  seen  all  around  the 


THE  MOON  AND  ECLIPSES  129 

disk  of  the  moon.  The  eclipse  is  then  called  annular,  because 
the  edge  of  the  sun  is  seen  as  a  ring  (annulus,  a  ring). 

If  the  moon  passes  the  sun  in  the  position  C  or  D,  there 
will  be  a  large  or  a  small  partial  eclipse,  as  the  moon  passes. 

If  the  sun  is  far  from  the  node,  as  at  E,  the  moon  will  pass 
clear  of  the  sun  and  there  will  be  no  eclipse  at  all. 

As  the  moon's  shadow  and  penumbra  pass  over  the  earth, 
the  diameter  of  the  penumbra  is  a  little  more  than  half  that 
of  the  earth.  Hence  the  sun  will  never  be  eclipsed  all  over 
the  earth,  but  only  in  those  parts  over  which  the  penumbra  or 
shadow  sweeps.  If  an  observer  in  one  part  of  the  earth  saw 
a  central  eclipse,  like  J3,  an  observer  farther  south  would  see 
the  moon  pass  north  of  the  sun's  center,  as  in  (7,  D,  E,  and  there 
would  be  a  large  partial  eclipse,  a  small  one,  or  no  eclipse  at 
all,  according  to  his  position. 

Of  course  every  eclipse  of  the  sun  begins  by  being  small 
and  partial,  and  gradually  increases  as  the  opaque  body  of 
the  moon  advances.  The  various  appearances  of  the  eclipse 
as  it  advances  are  called  phases  of  the  eclipse. 

A  total  eclipse  of  the  sun  is  a  very  impressive  sight,  espe- 
cially if  one  observes  it  from  a  high  elevation  where  he  can 
see  many  miles  around.  Owing  to  the  direction  of  the  moon's 
motion  in  her  orbit  the  shadow  of  the  moon  sweeps  along  the 
earth  in  an  easterly  direction ;  it  may  be  due  east,  or  it  may 
incline  to  the  north  or  south  in  a  greater  or  less  degree. 
During  the  partial  phase  of  such  an  eclipse  the  observer  will 
see  nothing  very  striking  except  the  gradual  covering  up  of 
the  sun's  disk,  reducing  it  to  the  form  of  a  crescent.  When 
it  is  almost  covered,  if  he  looks  in  the  direction  from  which 
the  shadow  is  coming,  he  will  see  the  darkness  approaching, 
perhaps  at  the  rate  of  nearly  a  mile  a  second.  As  the  shadow 
reaches  him  the  sun  entirely  disappears.  Looking  up,  he  now 
sees  the  black  body  of  the  moon  with  the  sun's  corona  around 
it.  The  latter  is  a  most  beautiful  effulgence,  like  the  glory 
which  the  old  painters  depicted  round  the  heads  of  their  saints. 
It  is  quite  irregular  in  shape,  parts  of  it  extending  out  in 

NEWCOMU'S    ASTRON.  9 


130  ASTRONOMY 

streamers.  It  shades  off  so  gradually  that  no  distinct  outline 
can  be  seen.  When  viewed  with  a  telescope  the  corona  is 
seen  to  have  a  somewhat  woolly  or  fibrous  structure,  which  is 
also  shown  on  the  photographs. 


Fio.  ?6.  —  The  solar  corona  during  a  total  eclipse  of  the  sun. 

The  light  of  the  corona  probably  comes  from  very  minute 
vaporous  particles  shot  up  from  the  sun,  and  perhaps  held  up 
by  some  form  of  magnetic  or  electric  action.  Part  of  the  light 
may  also  come  from  clouds  of  particles  revolving  round  the  sun. 
The  brightest  stars  will  also  be  visible.  It  will  not,  however, 
be  entirely  dark,  but,  as  described  by  Milton,  the  sun 

"...  From  behind  the  Moon 
.  .  o  disastrous  twilight  sheds. " 

This  is  because  the  sun  is  shining  through  the  air  all  around  the 
region  of  total  eclipse,  and  the  light  reflected  from  without 


THE  MOON  AND  ECLIPSES  131 

this  region  penetrates  the  whole  shadow,  and  enables  us  to  see 
surrounding  objects.  The  darkness  is  about  that  which  we 
have  half  an  hour  after  sunset.  The  time  by  a  watch  can  be 
seen  during  the  whole  of  the  eclipse. 


FJG.  76.  —  Another  view  of  the  solar  corona. 

8c  Recurrence  of  Eclipses.  —  Since  there  are  two  opposite 
nodes  of  the  moon's  orbit,  and  the  sun  makes  an  apparent  cir- 
cuit of  the  heavens  in  the  course  of  a  year,  it  follows  that  the 
sun  will  appear  to  pass  the  moon's  nodes  twice  in  every  year, 
at  an  interval  of  about  six  months.  Hence  eclipses  of  the  sun 
or  moon  may  occur  at  two  opposite  times  of  the  year,  about 
six  months  apart.  We  may  call  these  times  eclipse  seasons. 

As  an  eclipse  may  occur  at  either  node  of  the  moon's  orbit, 
it  frequently  happens  that,  if  there  is  an  eclipse  of  the  moon  at 
one  node,  then,  when  she  makes  half  a  revolution,  which  takes 
about  fifteen  days,  there  will  be  an  eclipse  of  the  sun  at  the 
opposite  node,  and  vice  versa. 

If  the  position  of  the  moon's  nodes  were  invariable,  the 
eclipse  seasons  would  always  be  the  same,  year  after  year. 


132 


ASTRONOMY 


But  the  position  of  the  node  is  continually  changing,  owing  to 
the  attraction  of  the  sun  on  the  earth  and  moon.  The  manner 
in  which  this  change  takes  place  will  be  seen  by  studying 
figure  77.  Here  the  small  circles  show  ten  different  positions 
of  the  moon  as  she  is  passing  her  node,  the  position  of  her 
orbit  being  shown  by  the  dotted  line  mm  through  the  centers. 


In  the  next  to  the  last  position  she  is  exactly  at  the  node,  and 
is  therefore  crossing  the  ecliptic.  But  when  she  makes  a  revo- 
lution and  gets  back  to  the  same  position  in  the  heavens,  she 
will  not  follow  exactly  the  same  path,  but  will  follow  along 
the  line  nn.  In  another  revolution  she  will  pass  along  the 
line  oo,  and  so  on  continually.  At  the  fifth  revolution  the 
point  of  crossing  or  node  will  be  nearly  at  the  right  hand  end 
of  the  figure.  Hence  the  node  is  in  motion  from  east  toward 
west.  In  this  way  each  node  makes  a  complete  revolution  in 
the  heavens  in  eighteen  years  and  about  seven  months.  The 
result  of  this  is  that  the  eclipse  seasons  occur  about  twenty 
days  earlier  every  year  than  they  did  the  year  before,  because 
the  sun  in  its  apparent  patli  catches  up  to  the  node  that  much 
sooner  every  year. 


CHAPTER  IX 
THE  CALENDAR 

A  calendar  is  a  system  of  defining,  arranging,  and  numbering 
days,  months,  and  years,  so  as  to  form  a  continuous  measure  of 
time  to  be  used  by  the  people  of  a  country.  In  former  ages, 
when  people  were  not  in  so  close  communication  as  they  are 
now,  each  nation  generally  had  its  own  calendar.  But  at  the 
present  time  nearly  all  civilized  nations  have  adopted  the  one 
used  by  us. 

1.  Units  of  Time.  — The  first  and  most  natural  unit  of  time 
to  be  adopted  by  men  is  the  day ;  understanding  by  that  term 
the  period  between  two  successive  passages  of  the  sun  over 
the  meridian,  which,  as  we  have  already  explained,  is  slightly 
greater  than  the  true  time  of  one  revolution  of  the  earth  on  its 
axis.1  The  use  of  this  unit  of  time  was  enforced  upon  men  from 
the  beginning  by  the  alternation  of  the  activity  of  the  day 
with  the  repose  of  the  night. 

The  next  period  of  time  to  be  noted  was  the  year.  The 
cycle  of  the  seasons,  which  the  earliest  men  who  noticed  the 
order  of  nature  must  have  seen  to  be  due  to  the  varying  declina- 
tion of  the  sun,  determines  the  year.  One  of  the  earliest  works 
of  men  who  made  astronomical  observations  was  to  determine 
the  exact  times  at  which  the  sun  reached  the  equinoxes  in 

1  It  is  an  unfortunate  defect  of  our  language,  as  of  most  modern  languages, 
that  the  word  day  is  used  in  two  senses  :  (1)  the  period  during  which  the  sun 
is  above  the  horizon,  in  contradistinction  to  night  when  the  sun  is  below  the 
horizon ;  and  (2)  the  length  of  a  day  and  night  together.  The  reader  will,  in 
each  case,  see  for  himself  in  which  sense  the  term  is  used. 

133 


184  ASTRONOMY 

successive  years.  The  number  of  days  between  two  returns 
to  the  same  equinox  defined  the  length  of  the  year.  Very 
early  in  history  it  was  thus  discovered  that  the  length  of  the 
year  was  about  365  J  days. 

The  Month  and  Easter.  —  So  large  a  number  as  this  was 
inconvenient  to  keep  count  of.  An  intermediate  unit  was 
therefore  necessary.  This  was  afforded  by  the  changes  of  the 
moon.  At  intervals,  which  our  ancestors  found  to  be  about 
thirty  days,  the  moon  completed  the  circuit  of  the  heavens, 
disappeared  in  the  sun's  rays,  and  again  reappeared  in  the 
west  after  sunset.  The  reappearing  body  was  called  the  new 
moon.  The  interval  between  two  successive  new  moons  is  called 
the  lunar  month. 

In  very  ancient  times,  as  we  know  from  the  Bible  and  other 
writings,  the  moon  was  used  to  determine  the  times  of  certain 
religious  festivals.  This  practice  survives  with  us  in  the 
date  of  Easter,  which  is  determined  as  follows :  — 

The  first  full  moon  after  the  21st  of  March  in  every  year  is 
called  the  Paschal  full  moon.  Easter  Sunday  is  the  Sunday 
which  follows  this  full  moon.  If  the  latter  occurs  on  Sunday, 
Easter  is  the  Sunday  following. 

The  moon  goes  through  its  cycle  of  changes  a  little  more 
than  twelve  times  in  a  year.  Had  it  done  so  exactly  twelve 
times,  there  would  have  been  no  difficulty  in  using  it  as  a 
measure  of  months.  This  not  being  the  case,  it  was  impossible 
to  make  a  true  year  out  of  12  lunar  months.  A  year  thus 
measured  was  only  354£  days  in  length.  Those  who  used  it 
found  the  seasons  in  the  course  of  30  years  to  wander  through 
every  part  of  the  year,  which  was  inconvenient. 

Another  method  was  to  fit  the  lunar  months  and  years 
together,  a  year  having  sometimes  12  months  and  sometimes 
13.  This  also  was  inconvenient. 

A  third  method,  and  the  one  which  is  now  adopted  by  the 
leading  nations,  is  to  reject  the  lunar  months  altogether,  and 
divide  every  year  into  12  months,  without  any  respect  to  the 
moon. 


THE  CALENDAR  135 

2.  The  Julian  Calendar.  — Two  thousand  years  ago  the  Komans 
were  the  leading  nation  of  the  world ;  but  their  calendar  was 
in  great  confusion  before  the  time  of  Julius  Caesar,  because 
the  emperors  or  other  rulers  fixed  it  from  time  to  time  to  suit 
themselves.     Tp  remedy  this  confusion  Julius  Caesar  arranged 
what  is  now  known  after  him  as  the  Julian  Calendar.     He  (or 
his  learned  men)  saw  that  if  we  had  in  regular  succession  three 
years  of  365  days  and  then  one  year  of  366  days,  the  average 
length  of  the  year  would  be  365J  days,  which  was  then  sup- 
posed to  be  the  true  length.     So  this  plan  was  adopted  and 
was  carried  by  the  Komans  into  the  various  countries  which 
they  conquered.    Owing  to  its  approach  to  correctness  it  was 
continued  after  being  once  fully  accepted.    Thus  it  became  the 
calendar  of  civilized  Europe. 

3.  The  Gregorian  Calendar.  —  In  the  sixteenth  century  it  was 
found  that  the  period  of  365J  days  was-  a  little  more  than  the 
true  length   of   a  year.     The   error   was  not  very   great ;    it 
amounted  to  only  one  day  in  about  130  years.     Still,  it  had  the 
effect,  in  the  course  of  centuries,  of  making  the  equinox  fall  at 
a  different  time  of  the  year  from  that  at  which  it  had  been 
arranged  to  fall  by  the  festivals  of  the  Church.     In  the  six- 
teenth century  the  error  had  amounted  to  ten  days,  it  being 
assumed  that  the  correct  arrangement  was  that  made  by  the 
Council  of  Nice,  325  A.D.    To   remedy   this,   Pope   Gregory 
XIII,  in  1582,  modified  the  Julian  calendar  by  arranging  that 
the  closing  years  of  the  centuries  1600, 1700,  etc.,  should  not  be 
leap  years  unless  the  number  of  the  century  was  divisible  by 
four.     That  is  to  say,  1600,  2000,  2400,  etc.,  were  to  be  leap 
years  as  in  the  Julian  calendar,  but  1700,  1800,  1900,  2100, 
etc.,  were  to  be  common  years. 

This  is  called  the  Gregorian  Calendar,  after  the  name  of  the 
pope  who  established  it.  It  is  now  in  use  by  all  Christian 
nations  except  Russia  and  Greece.  Even  these  are  expected 
to  adopt  it  sometime. 

In  establishing  the  calendar  Gregory  added  ten  to  the  count 


136  ASTRONOMY 

of  days,  so  as  to  make  the  years  begin  at  the  same  time  they 
would  have  begun  had  his  calendar  been  adopted  by  the 
Council  of  Nice.  To  bring  this  about  it  was  ordered  that  the 
day  which,  in  the  Julian  calendar,  would  have  been  October 
5,  1582,  should  be  called  October  15,  so  that  the  15th  day  of 
that  particular  month  followed  the  4th  day.  This  made  a  dif- 
ference of  10  days  between  the  Gregorian  and  Julian  calendars, 
which  became  11  days  on  March  1,  1700,  because  in  the  Julian 
calendar  February  of  that  year  had  29  days,  whereas  it  only 
had  28  in  the  Gregorian  calendar.  In  1800  the  difference 
became  12  days,  and  from  1900  to  2100  it  will  be  13  days. 

Thus,  in  our  calendar,  the  rule  for  leap  year  is  that  every 
year  the  last  two  figures  of  whose  number  is  divisible  by  4  is 
a  leap  year,  except  the  terminal  years  of  a  century  ending  in 
00.  These  are  leap  years  when,  and  only  when,  the  number 
of  the  century  is  divisible  by  4. 

4.  The  Year.  —  The  reckoning  by  the  Julian  calendar  is  some- 
times called  Old  Style,  and  that  by  the  Gregorian  New  Style. 
Besides  the  length  and  arrangement  of  the  year,  a  calendar 
must  determine  two  things :  at  what  time  of  the  period  of  365 
or  366  days  a  new  year  shall  begin ;  and  from  what  epoch  the 
years  shall  be  counted.  Even  after  the  adoption  of  the  Julian 
and  Gregorian  calendars,  there  was  some  difference  among 
nations  as  to  the  beginning  of  the  year.  In  England  it  com- 
menced March  1  instead  of  January  1.  The  change  to  the 
latter  date  was  made  in  1752,  and  is  now  universal. 

In  ancient  times  it  was  the  custom  to  count  years  from  the 
accession  of  some  monarch,  or  the  foundation  of  the  govern- 
ment, or  of  its  capital  city.  Thus  the  Komans  counted  their 
years  from  the  supposed  foundation  of  the  city  of  Kome.  We 
see  a  survival  of  this  custom  among  us  when,  in  official  docu- 
ments, the  year  of  the  Independence  of  the  United  States  is 
given.  But  for  the  purposes  of  civilized  life,  our  practice  of 
counting  the  years  from  the  birth  of  Christ  has  become  coex- 
tensive with  the  Julian  and  Gregorian  calendars. 


THE  CALENDAR  137 

&  Features  of  the  Church  Calendar.  —  In  our  religious  festi- 
vals there  still  survive  some  remains  of  the  ancient  attempts 
to  arrange  the  measure  of  time  by  the  moon.  One^of  these  is 
the  Metonic  Cycle,  called  after  Me  ton,  a  Greek  who  lived  about 
433  B.C.  He  found  that  a  period  or  cycle  of  6940  days  could 
be  divided  up  into  235  lunar  months,  or  19  solar  years. 

In  consequence  of  19  years  being  nearly  an  exact  number  of 
lunar  months,  Easter  will  commonly,  though  not  universally, 
fall  upon  the  same  day  after  a  period  of  19  years.  Hence,  if 
we  count  the  years  from  1  to  19,  and  then  begin  over  again, 
the  dates  of  Easter  will  be  repeated  in  regular  order.  This 
number,  which  ranges  from  1  to  19,  is  called  the  Golden 
number.  It  is  said  to  owe  its  name  to  the  enthusiasm  of  the 
Greeks  over  Meton's  discovery,  who  caused  the  division  and 
numbering  of  the  years  on  the  plan  of  Meton  to  be  inscribed 
on  public  monuments  in  letters  of  gold.  The  rule  for  finding 
the  golden  number  is  to  divide  the  number  of  the  year  by  19 
and  add  1  to  the  remainder.  It  is  employed  for  finding  the 
time  of  Easter  Sunday. 

In  our  Church  calendars  a  system  is  used  for  indicating  the 
day  of  the  week  on  which  any  given  date  will  fall.  January  1 
is  marked  by  the  letter  A,  January  2  by  B,  and  so  on  to  G, 
when  the  letters  begin  over  again,  and  are  repeated  through 
the  year  in  the  same  order.  Thus  each  letter  in  any  one 
year  indicates  the  same  day  of  the  week  through  the  yea,r, 
except  in  leap  years,  when  February  29  and  March  1  are 
marked  by  the  same  letter,  so  that  a  change  occurs  at  the 
beginning  of  March.  The  letter  corresponding  to  Sunday  of 
any  year  is  called  the  Dominical,  or  fiunday  letter  for  that 
year.  When  we  once  know  what  letter  it  is,  all  the  Sundays 
of  the  year  are  indicated  by  it.  In  leap  years  there  are 
two  dominical  letters,  one  extending  to  the  end  of  Febru- 
ary, and  the  other  through  the  remaining  ten  months  of  the 
year. 

We  shall  find  by  making  the  calculation  that  28  Julian 
years  contain  exactly  1461  weeks :  It  follows  that  at  the  end 


138  ASTRONOMY 

of  28  years  the  dominical  letter  will  be  repeated  as  before. 
This  period  of  28  years  is  called  the  solar  cyde. 

Note  that 

28  x  366J  =  10227  =  7  x  1461. 

An  exception,  however,  occurs  when  we  pass  over  a  centen- 
nial year  which  is  not  a  leap  year,  as  in  1900.  For  example, 
the  dominical  letter  will  not  be  the  same  in  1909  as  it  was  in 
1890,  19  years  before.  But  the  solar  cycle  will  go  on  regularly 
through  the  twentieth  and  twenty-first  centuries,  a  break  again 
occurring  in  the  year  2100. 

6.  The  Hours.  —  In  ancient  times  the  period  from  sunrise  to 
sunset  was  divided  into  12  hours,  which  were  counted  from  1 
to  12.  Thus  men  spoke  of  the  first  hour  of  the  day,  meaning 
the  first  hour  after  sunrise,  the  second,  etc.,  a  system  quite 
familiar  to  readers  of  the  Scriptures.  The  third  hour  was 
that  when  the  middle  of  the  forenoon  was  approaching ;  the 
sixth  hour  terminated  at  noon ;  the  ninth  hour  terminated  at 
the  middle  of  the  afternoon,  and  the  twelfth  hour  at  sunset. 
The  night  was  also  divided  in  the  same  way  into  12  hours : 
the  first  hour  of  the  night,  the  second,  etc.,  to  the  twelfth. 

The  day  being  longer  in  summer  than  in  winter,  the  length 
of  the  hours  thus  defined  changed  with  the  seasons,  and  the 
night  hours  were  shortest  when  the  day  hours  were  longest. 
As  people  had  no  clocks  or  watches  in  those  days,  and  no 
exact  instruments  for  measuring  time  were  kept  in  the  house, 
this  variability  of  the  length  of  the  hours  did  not  cause  any 
serious  inconvenience.  If  the  hours  had  been  of  equal  length, 
this  system  of  counting  12  hours  of  the  day  and  then  12  hours 
of  the  night  would  have  been,  in  some  respects,  more  conven- 
ient than  our  own,  because  the  count  of  hours  would  have 
gone  on  continuously  through  the  day,  and  again  through  the 
night.  But  the  practice  of  beginning  a  new  day  at  sunset  was 
found  to  be  inconvenient,  because  our  activities  always  con- 
tinue into  the  night ;  and  confusion  would  arise  from  passing 
from  one  day  to  another  at  6  o'clock  in  the  evening. 


CALENDAR  139 

When  the  day  was  taken  to  begin  at  midnight,  it  was  in 
some  places  the  practice  to  count  the  hours  from  1  to  24. 
Then  during  the  forenoon  the  count  of  hours  would  have  been 
the  same  as  that  we  use  up  to  noon.  But  what  we  call  1 
o'clock  would  have  been  13  o'clock,  and  so  on  to  24  o'clock, 
which  would  have  occurred  at  midnight. 

Probably  men  found  it  inconvenient  to  carry  on  a  count  of 
hours  greater  than  12.  In  consequence  the  practice  was  intro- 
duced of  beginning  the  count  of  hours  over  again  at  noon,  as 
we  do,  and  indicating  which  of  the  12-hour  periods  was  meant 
by  the  words  A.M.  and  P.M.,  abbreviations  of  the  terms  ante 
meridiem  and  post  meridiem.  This  beginning  of  a  new  count 
of  12  hours  at  noon  is  frequently  an  inconvenience.  Hence 
efforts  are  being  made  in  some  quarters  to  induce  people  to 
count  the  hours  up  to  24.  On  the  Italian  and  some  Canadian 
railways  this  is  done  in  the  time-tables,  and  it  would  avoid 
much  trouble  if  it  were  done  everywhere,  and  if  we  all  counted 
the  hours  up  to  24  from  one  midnight  to  the  next. 

Astronomers  count  the  hours  from  0  to  24  on  the  system  we 
have  just  suggested,  only  their  day  begins  at  noon  instead  of 
midnight.  This  is  because  they  use  the  day  solely  as  a  meas- 
ure of  time,  without  regard  to  light  or  darkness,  and  it  is  just 
as  convenient  to  begin  it  at  one  moment  as  at  another.  Its 
beginning  being  determined  by  the  passage  of  the  mean  sun 
across  the  meridian,  it  is  more  natural  to  count  the  hours  from 
that  moment.  Thus,  in  astronomical  reckoning,  mean  noon  is 
0  hour.  What  we  call  1  o'ctock  P.M.  is  1  hour,  and  so  on  to 
midnight,  which  is  12  hours.  Then  1  o'clock  in  the  morning 
is  13  hours,  and  so  on  to  11  o'clock  A.M.,  which  is  23  hours. 

This  mode  of  beginning  the  day  and  counting  the  hours  is 
called  astronomical  time,  while  the  count  on  the  ordinary  plan 
is  called  civil  time. 


CHAPTER  X 


GENERAL  PLAN  OF  THE   SOLAR  SYSTEM. 

1.  Orbits  of  the  Planets.  —  We  explained  in  the  second  chap- 
ter that  the  principal  bodies  of  the  solar  system  are  the  sun 
and  a  number  of  planets  revolving  around  it,  on  one  of  which 
we  dwell.  We  have  now  to  learn  some  general  facts  about 
the  arrangement  and  motions  of  the  planets. 


FIG.  78.  —  Showing  how  an  ellipse  may  be  drawn. 

The  orbits  of  the  planets,  including  that  of  the  earth,  are 
not  exact  circles,  but  ellipses,  which,  however,  differ  so  little 
from  circles  that  the  eye  could  not  see  the  deviation. 

An  ellipse  may  be  described  by  attaching  the  ends  of  a  string 
to  two  fixed  points,  E  and  F,  whose  distance  apart  is  less  than 
the  length  of  the  string.  Then  by  passing  a  pencil  around  the 

140 


GENERAL  PLAN  OF  THE  SOLAR  SYSTEM         141 

string,  keeping  the  latter  tight,  the  point  of  the  pencil  will 
describe  an  ellipse.  Each  of  the  points  E  and  F,  around  which 
the  ellipse  is  described,  is  called  a  focus.  The  center  C  of  the 
ellipse  is  half  way  between  the  foci.  The  longest  diameter, 
ABj  is  called  the  major  axis;  the  shortest,  MN9  the  minor 
axis. 

The  distance  FC  or  CE  between  the  center  and  each  focus 
was  formerly  called  the  eccentricity  of  the  ellipse.  In  modern 
times  we  call  the  eccentricity  the  quotient  of  this  distance 
divided  by  the  major  axis.  It  is  commonly  expressed  as  a 
decimal  fraction. 

2.  Kepler's  Laws.  —  The  laws  of  motion  of  the  planets 
round  the  sun  are  called  Kepler's  laws,  after  the  astronomer 
Kepler  who  discovered  them. 

The  radius  vector  of  a  planet  is  the  line  from  the  sun  to  the 
planet.  As  the  planet  moves  round  the  sun  the  radius  vector 
is  conceived  to  turn  round  the  sun  with  it,  as  on  a  pivot. 

The  mean  distance  of  a  planet  from  the  sun  is  half  the  sum 
of  its  greatest  and  least  distances.  It  is  equal  to  one  half  the 
major  axis  of  the  orbit. 

The  periodic  time  of  a  planet  is  the  time  it  takes  to  make  a 
revolution  round  the  sun. 

Kepler's  laws  are  these :  — 

I.  The  orbit  of  each  planet  is  an  ellipse,  having  the  sun  in  one 
focus. 

II.  As  the  planet  moves  round  the  sun  the  radius  vector  sweeps 
over  equal  areas  in  equal  times. 

III.  The  squares  of  the  periodic  times  of  the  planets  are  propor- 
tional to  the  cubes  of  their  mean  distances  from  the  sun. 

To  understand  the  second  law,  suppose  figure  79  to  be  the 
orbit  of  a  planet  having  the  sun  in  the  focus.  Mark  on  the 
orbit  the  points  P,  Q,  R,  etc.,  which  the  planet  reaches  at 
equally  distant  intervals  of  time,  —  it  may  be  intervals  of  a 
day,  a  month,  or  a  year.  Draw  the  radii  vectors  from  the  sun 


142  ASTRONOMY 

to  each  point.     Then  the   triangular   areas  PSQ,  QSR,  etc., 
described  by  the  radius  vector  will  all  be  equal  to  each  other. 

It  follows  from  this  law  that  the  nearer  the  planet  is  to  the 
sun  the  faster  it  moves.  We  have  already  explained  this  by 
showing  that,  as  the  planet  falls  toward  the  sun  it  gains  veloc- 
ity, and  as  it  recedes  from  the  sun  it  loses  velocity. 


FIG.  79.  —  Showing  how  the  radius  vector  of  a  planet,  as  the  latter 
moves  round  the  sun,  sweeps  over  equal  areas  in  equal  times. 

If  we  look  at  figures  78  and  79  we  shall  see  that  because  the 
sun  is  not  in  the  center  of  an  orbit,  but  in  one  focus,  there  are 
in  every  orbit  two  opposite  points  at  the  ends  of  the  major 
axis,  at  one  of  which  the  planet  is  nearest  the  sun,  and  the 
other  farthest  away. 

The  point  of  the  orbit  where  the  planet  comes  nearest  the 
sun  is  called  the  perihelion;  the  point  where  it  is  farthest 
away  is  called  the  aphelion. 

3.  Structure  of  the  Solar  System.  —  The  eight  principal  plan- 
ets of  the  solar  system  are  called  major  planets  to  distinguish 
them  from  an  immense  group  of  smaller  ones  called  minor  planets. 
All  the  planets,  except  the  two  nearest  the  sun,  have  one  or 
more  moons  or  satellites  revolving  round  them.  Thus  we  have 


GENERAL  PLAN   OF  THE  SOLAB  SYSTEM        143 

to  consider,  not  merely  planets  revolving  round  the  sun,  but 
systems  each  composed  of  a  planet  and  its  satellites,  fashioned 
somewhat  after  the  solar  system.  As  the  sun  is  the  center 


SUN 


FIG.  80.  —  Showing  the  relative  size  of  the  sun  and  the  principal  planets. 

around  which  the  planets  revolve,  so  are  some  of  the  planets 
centers  round  which  satellites  revolve.  The  satellites  are 
generally  much  smaller  than  their  central  planets.  Our  moon 


144  ASTRONOMY 

is  a  satellite  of  the  earth,  revolving  round  it  in  the  manner 
we  have  already  explained. 

A  planet  having  a  satellite  moving  round  it  is  called  a  pri- 
mary planet  to  distinguish  it  from  the  satellites,  which  are 
also  called  secondary  planets. 

The  time  in  which  a  planet  completes  its  revolution  round 
the  sun,  or  a  satellite  round  its  primary  planet,  is  called  its 
periodic  time,  or  its  period. 

In  addition  to  the  major  planets  and  their  satellites,  there 
is  a  curious  group  of  bodies  called  minor  planets  or  asteroids 
occupying  a  region  where  we  might  suppose  there  ought  to  be 
a  single  planet. 

The  following  is  a  list  of  the  principal  bodies  or  groups  of 
bodies  of  the  solar  system,  with  the  periods  of  revolution 
of  the  planets  round  the  sun :  — 

1.  The  Sun,  the  great  central  body. 

2.  The  planet  Mercury         ....  Period  88  days. 

8.  The  planet  Venus Period  225  days. 

4.   The  planet  Earth,  with  1  satellite  .         .  Period  1  yeaf. 

6.   The  planet  Mars,  with  2  satellites  . .       .        Period  2  years. 

6.  A  group  of  several  hundred  minor  planets, 

or  asteroids,  periods  mostly  from  .  3  to  6  years. 

7.  The  planet  Jupiter,  with  6  satellites  .  Period  12  years. 

8.  The  planet  Saturn,  with  8  satellites  .  Period  29  years. 

9.  The  planet  Uranus,  with  4  satellites  .  Period  84  years. 
10.   The  planet  Neptune,  with  1  satellite  .  Period  165  years. 

The  planets  Mercury  anjj.  Venus,  which  revolve  inside  the 
orbit  of  the  earth,  are  called  inferior  planets. 

Those  whose  orbits  are  outside  that  of  the  earth  are  called 
superior  planets. 

4.  Distances  of  the  Planets;  Bode's  Law.  —  The  distances  of 
the  planets  from  the  sun  range  from  36  million  miles  in  the 
case  of  Mercury,  to  2775  million  in  the  case  of  Neptune.  The 
distance  of  the  earth  is  93  million  miles.  But  astronomers  do 
not  use  miles  in  celestial  measurement,  not  only  because  they 
are  too  short,  but  because  distances  in  miles  cannot  be  always 


GENERAL  PLAN  OF  THE  SOLAR  SYSTEM 


145 


known  exactly  and  other  units  of  measurement  are  more  con- 
venient. To  express  distances  of  the  planets  they  take  the 
distance  of  the  earth  from  the  sun  as  the  unit  of  measurement. 
We  may  call  this  unit  a  sun-distance.  Expressed  in  terms  of 
sun-distances,  the  distance  of  Mercury  is  0.387,  that  of  the 
Earth  1,  and  that  of  Neptune  30. 

Bode's  Law.  —  About  a  hundred  years  ago  it  was  found  by  the 
astronomer  Bode  that  the  distances  of  the  planets  then  known 
could  be  found  approximately  in  the  following  way :  — 

Form  the  row  of  numbers  0,  3,  6, 12,  each  one  after  the  second 
being  found  by  multiplying  the  preceding  one  by  2.  Then 
add  4  to  each  number,  and  divide  the  sum  by  10,  thus :  — 


0 

3 

6 

12 

24 

48 

96 

192 

384 

4 

4 

4 

4 

4 

4 

4 

4 

4 

4 

7 

10 

16 

28 

62 

100 

196 

388 

10  0.4 

0.7 

1.0 

1.6 

2.8 

5.2 

10.0 

19.6 

38.8 

These  numbers  may  now  be  compared  with  actual  distances 
of  the  planets  from  the  sun,  as  follows :  — 


True 
Dist. 

Bode'  s 
Law 

Error 
of  Law 

Period 
in  Years 

0.387 

0.4 

.013 

0.2408 

Venus      .  .      .  .  •  • 

0.723 

0.7 

.023 

06152 

Earth    

1.000 

1.0 

.000 

1.000 

Mars     

1.524 

1.6 

.076 

1  881 

Blank    

2.8 

5.203 

5.2 

.003 

11.86 

9.539 

10.0 

.461 

29.46 

Uranus  .         

19.18 

19.6 

.42 

83.74 

Neptune         . 

30.04 

38  8 

8  76 

164  78 

Except  for  the  planet  Neptune,  which  was  not  known  when 
Bode  lived,  the  numbers  found  by  his  rule  hit  very  near  the 
truth,  as  we  see  by  the  errors  given  in  the  third  column.  The 
most  interesting  feature  of  the  table  is  that  Bode  had  to  fill  a 


NEWCOMB'S  ASTRON.  — 10 


146  ASTRONOMY 

wide  gap  between  Mars  and  Jupiter,  where  Ms  rule  would 
place  a  planet,  but  where,  in  his  time,  no  planet  was  known  to 
exist.  He  therefore  predicted  that  a  planet  might  some  time 
be  found  in  this  gap.  Not  only  one,  but  hundredsjvhave  been 
found  since  his  time. 

In  1846,  when  Neptune  was  discovered,  it  was  seen  that  its 
distance  did  not  agree  with  the  rule.  This  shows  that  what  is 
called  Bode's  law  is  not  really  a  law  of  nature,  but  only  an 
accidental  coincidence. 

We  have  added  the  more  exact  periodic  times  to  the  above 
table  that  the  student  may  test  Kepler's  third  law.  Find  the 
cubes  of  the  several  distances  and  the  squares  of  the  periods 
and  compare  the  one  with  the  other. 

5.  Aspects  of  the  Planets.  —  In  consequence  of  our  being 
carried  around  the  sun  upon  the  earth,  the  apparent  motions 
of  the  planets  are  different  from  their  real  motions.  The 
varying  positions  of  the  planets  relative  to  each  other  and  to 
the  sun  are  called  aspects.  To  explain  these  various  aspects, 
and  show  their  cause,  certain  terms  are  used  which  we  shall 
now  define. 

The  elongation  of  a  planet  is  its  apparent  angular  distance 
from  the  sun.  The  word  is  generally  applied  to  the  elongation 
of  Mercury  or  Venus.  In  figure  81  let  the  earth  be  in  the 
position  shown,  and  let  the  circle  represent  the  orbit  of 
Mercury  or  Venus  around  the  sun.  Let  P  be  any  position 
of  the  planet  in  its  orbit.  Then  the  angle  between  the  lines 
ES  drawn  from  the  earth  to  the  sun,  and  EP  drawn  from  the 
earth  to  the  planet,  is  the  elongation  of  the  planet. 

The  elongation  is  greatest  when  the  planet  is  at  one  of  the 
points  M  or  N.  At  M  it  will  appear  east  of  the  sun,  and  is 
then  said  to  be  at  its  greatest  east  elongation.  In  this  case  it 
will  be  visible  after  sunset. 

When  the  planet  is  at  N9  it  is  said  to  be  at  its  greatest 
west  elongation.  It  may  then  be  seen  in  the  morning  before 
sunrise. 


GENERAL  PLAN   OF  THE  SOLAR   SYSTEM         147 

When  two  heavenly  bodies,  in  their  courses  around  the  celes- 
tial sphere,  pass  by  each  other,  they  are  said  to  be  in  conjunc- 
tion. 

When  they  are  on  opposite  sides  of  the  celestial  sphere,  so 
that,  for  example,  one  would  be  rising  while  the  other  was 
setting,  they  are  said  to  be  in  opposition. 


FIG.  81.  —  Showing  the  different  directions  in  which  an  inferior  planet 
may  be  seen  from  the  earth.  The  greatest  elongation  is  the  angle 
between  the  lines  so  marked  and  the  line  drawn  from  the  earth  to 
the  sun. 

The  conjunction  of  a  planet  with  the  sun  is  called  inferior, 
when  the  planet  passes  between  the  sun  and  the  earth.  It  is 
called  superior  when  the  planet  is  beyond  the  sun. 

It  is  evident  that  the  superior  planets  can  never  be  in  inferior 
conjunction  with  the  sun  because  they  can  never  pass  between 
the  earth  and  the  sun.  The  inferior  planets  Mercury  and 
Venus  may  be  either  in  inferior  or  superior  conjunction. 

6.  Apparent  Motions  of  the  Planets.  —  If  we  could  view  the 
stars  and  planets  from  the  sun,  each  star  would  always  appear 
fixed  in  the  same  place,  and  the  planets  would  be  seen  always 
moving  forward  in  their  orbits,  completing  their  revolution  in 
the  times  we  have  mentioned.  But  when  we  consider  the 
apparent  motions,  as  we  see  them  from  the  earth,  we  find  that 


148  ASTRONOMY 

they  are  affected  not  only  by  these  real  motions,  but  by  an 
apparent  swing  caused  by  our  being  carried  around  the  sun 
upon  the  earth. 

After  making  a  long  slow  sweep  toward  the  east  for  several 
months,  or  perhaps  a  year,  the  planet  will  gradually  stop  and 
make  a  short  sweep  toward  the  west.  Then  it  will  gradually 
stop,  and  again  start  on  a  long  sweep  eastward,  and  so  on. 

The  sweep  of  a  planet  from  west  toward  east  is  called 
direct  motion. 

The  sweep  from  east  toward  west  is  called  retrograde  motion. 

Between  the  east  and  west  sweeps  the  planet  seems  nearly 
at  rest,  it  is  then  said  to  be  stationary. 

Apparent  Motion  of  a  Superior  Planet.  —  To  show  how  the 
motion  of  the  earth  causes  an  apparent  retrograde  motion  of  a 
planet,  let  EF  in  figure  82  represent  the  orbit  of  the  earth,  and 


FIG.  82. — Showing  the  different  directions  in  which  a  superior  planet 
may  be  seen  in  consequence  of  the  motion  of  the  earth.  As  the 
earth  swings  along  from  E  toward  F  it  moves  faster  than  the  planet 
so  that  in  the  middle  of  the  swing  the  planet  appears  to  have  a 
motion  in  the  opposite  direction. 

the  arrows  the  direction  of  its  motion  around  the  sun.  Sup- 
pose a  body  to  be  at  rest  at  the  point  P.  Then,  when  the  earth 
is  at  E,  the  body  will  be  seen  on  the  celestial  sphere  in  the  direc- 
tion ES.  When  the  earth  reaches  the  point  F  the  body  will 


GENERAL  PLAN   OF  THE  SOLAR  ^SYSTEM        149 

be  seen  on  the  celestial  sphere  in  the  direction  FR.  That  is  to 
say,  while  the  earth  has  moved  from  E  to  F,  the  body,  though 
really  at  rest,  has  seemed  to  swing  on  the  celestial  sphere 
through  an  arc  equal  to  the  angle  EPF.  This  apparent  motion, 
being  the  opposite  of  that  of  the  earth,  will  be  retrograde. 

But  if  P  is  not  a  fixed  body,  but  a  planet,  it  is  really  in  mo- 
tion in  the  same  direction  as  the  earth.  If  it  moved  as  fast  as 
the  earth,  or  faster,  there  would  be  no  retrograde  swing  at  all. 
But  a  superior  planet  moves  more  slowly  than  the  earth. 
Hence  there  is  some  retrograde  motion,  though  less  than  there 
would  be  were  the  planet  at  rest. 

As  the  earth  is  moving  through  the  right  hand  arc  of  its 
orbit,  from  F  to  J57,  the  direct  motion  of  the  planet  as  we  see  it 
is  increased  by  the  effect  of  the  earth's  motion,  so  as  to  appear 
greater  than  it  really  is.  Hence  the  planet  makes  a  long 
direct  swing. 

We  may,  therefore,  sum  up  the  matter  by  saying  that  there 
is  a  real  direct  motion  of  each  superior  planet  around  the 
celestial  sphere  corresponding  to  its  motion  around  the  sun, 
and  that  the  apparent  deviations  from  this  real  motion  are 
in  the  nature  of  swings  due  to  the  revolution  of  the  earth  on 
which  we  live  around  the  sun. 

Apparent  Motions  of  the  Inferior  Planets.  —  These  planets  have 
direct  and  retrograde  swings  like  the  superior  ones,  but  for  rea- 
sons a  little  different.  If  such  a  planet  were  alongside  the  sun, 
the  effect  of  the  annual  revolution  of  the  earth  would  be  that 
the  sun  would  seem  to  us  to  carry  the  planet  with  it  in  its 
apparent  annual  motion  round  the  celestial  sphere.  Hence, 
the  effect  of  the  earth's  motion  is  to  make  the  inferior  planets 
complete  an  apparent  revolution  round  the  sphere  in  a  year. 

Because  these  planets  revolve  around  the  sun  they  seem  to 
us  to  swing  first  to  one  side  of  the  sun  and  then  to  the  other. 
Hence  they  have  apparent  direct  and  retrograde  motions,  as  in 
the  case  of  the  superior  planets.  During  the  retrograde  swing 
the  planet  seems  to  pass  the  sun  from  east  toward  west ;  during 
the  direct  swing  it  passes  from  the  west  to  the  east  of  the  sun. 


150  ASTRONOMY 

7.  Perturbation  of  the  Planets.  —  If  a  planet  were  attracted 
by  no  other  body  than  the  sun,  it  is  found  that  it  would 
move  around  the  sun  in  an  ellipse  in  exact  accordance  with 
Kepler's  laws.  This  ellipse  would  remain  in  the  same  position 
forever. 

But,  according  to  the  law  of  universal  gravitation,  each 
planet  is  attracted,  not  only  by  the  sun,  but  by  all  other  planets. 
In  consequence  of  this  attraction  the  motion  of  each  planet 
deviates  from  the  fixed  orbit  that  it  would  describe  if  the  sun 
alone  attracted  it.  These  deviations  are  called  perturbations. 

In  consequence  of  the  perturbations,  each  planet  is  some- 
times a  little  ahead  of  the  place  it  would  otherwise  occupy 
and  sometimes  a  little  behind  it;  sometimes  a  little  outside 
the  orbit  and  sometimes  a  little  inside.  The  average  orbit 
which  the  planet  describes  is  also  subject  to  slow  changes 
which  are  called  secular  variations.  This  term  is  applied 
because  these  variations  continue  through  many  ages.  Thus 
the  eccentricity  of  the  earth  has  been  diminishing  for  many 
thousand  years  and  will  continue  to  diminish  for  many  thou- 
sand years  to  come.  In  fact  the  eccentricities  of  the  orbits  of 
all  planets  are  changing  in  this  way,  some  increasing  and  others 
diminishing.  The  position  of  the  perihelia  of  the  earth  and 
all  the  planets  is  slowly  changing.  The  most  rapid  changes 
occur  in  the  case  of  the  moon.  It  is  owing  to  the  attraction 
of  the  sun  that  the  line  of  nodes  makes  a  revolution  in  18 
years,  and  that  the  perigee  moves  round  in  9  years. 

Ever  since  the  time  of  Newton  many  of  the  ablest  mathe- 
maticians in  the  world  have  investigated  the  laws  according 
to  which  these  deviations  in  the  motions  of  the  planets  take 
place.  The  results  they  have  reached  form  some  of  the  most 
remarkable  triumphs  of  the  human  intellect. 

One  result  is  that,  by  means  of  tables  of  motions  of  the 
planets,  the  astronomer  can  compute  these  motions  for  many 
centuries  past  or  future,  with  such  exactness  that  if  the  com- 
puted planet  were  placed  along  side  the  real  one,  the  keenest 
eye  could  not  distinguish  between  the  two. 


CHAPTER  XI 
THE   INNER  GROUP  OF  PLANETS 

IF  we  examine  the  list  of  the  eight  major  planets  in  §  3  of 
the  preceding  chapter  we  shall  see  that  they  may  be  divided 
into  two  groups.  One  group  comprises  the  four  inner  ones, 
Mercury,  Venus,  the  Earth,  and  Mars ;  the  other,  the  four  outer 
ones,  Jupiter,  Saturn,  Uranus,  and  Neptune.  The  groups  are 
separated  by  the  region  of  minor  planets.  The  outer  group  is 
distinguished  by  the  great  size  as  well  as  the  great  distance  of 
the  planets  that  compose  it.  In  this  chapter  we  shall  describe 
the  inner  group,  and  that  of  the  asteroids. 

1.  The  Planet  Mercury.  —  Mercury,  the  nearest  planet  to  the 
sun,  is  much  the  smallest  of  all  the  major  planets.  Its  revo- 
lution being  completed  in  88  days,  it  makes  more  than  four 
revolutions  a  year. 

Synodic  Revolution  of  Mercury.  —  Suppose  that  at  a  certain 
time,  the  earth  is  at  E,  figure  83,  and  Mercury  at  M.  The 
latter  is  then  in  inferior  conjunction.  At  the  end  of  88  days  it 
will  have  made  a  complete  revolution  and  got  back  to  the  point 
M.  But  it  will  not  then  be  in  inferior  conjunction,  because 
the  earth  will  have  moved  forward  in  its  orbit.  When  it 
catches  up  to  the  earth,  so  as  to  be  again  in  inferior  conjunc- 
tion, it  will  be  at  N  and  the  earth  at  F.  The  planet  is  then 
said  to  have  made  a  synodic  revolution.  The  synodic  period 
of  Mercury  is  116  days,  or  a  little  less  than  4  months. 

When  Mercury  is  near  inferior  conjunction,  it  is  invisible  in 
consequence  of  the  brightness  of  the  sun's  rays.  A  few  days 


152 


ASTRONOMY 


later  it  will  have  passed  around  so  far  as  to  be  visible  with  a 
telescope  west  of  the  sun.  A  month  later  it  will  be  near  its 
greatest  western  elongation,  and  can  then  be  seen  with  the 
naked  eye,  in  the  east,  before  sunrise.  In  another  month  it 
will  be  on  the  other  side  of  the  sun  in  superior  conjunction, 
and  again  invisible.  A  month  later  it  will  be  visible  in  the 
west,  after  sunset. 


FIG.  83.  —  Showing  the  synodic  period  of  Mercury. 

Visibility  of  Mercury.  —  Near  greatest  elongation  Mercury  is 
very  plainly  visible  to  the  naked  eye,  as  it  is  then  brighter 
than  any  of  the  fixed  stars.  If,  looking  in  the  west  after  sun- 
set, you  see  a  star  near  the  horizon  in  the  red  glow  of  twilight, 
you  may  suppose  it  very  likely  the  planet  Mercury.  But,  to 
be  sure,  you  must  know  the  position  of  Venus  and  of  the 
brighter  fixed  stars.  If  the  object  can  neither  be  Venus  nor  a 
fixed  star,  it  is  Mercury. 

Nodes  and  Transits  of  Mercury.  —  If  the  orbit  of  Mercury 
were  in  the  plane  of  the  ecliptic,  it  would  pass  between  the 
the  sun  and  the  earth  at  every  inferior  conjunction.  We  should 


THE  INNER  GROUP  OF  PLANETS  153 

then  see  the  planet  as  a  small  round  spot  on  the  sun's  disk, 
invisible  to  the  naked  eye,  but  easily  observed  with  a 
telescope. 

But  as  a  matter  of  fact,  the  orbit  of  the  planet  is  inclined  7° 
to  the  ecliptic  ;  hence,  at  nearly  every  inferior  conjunction  the 
planet  passes  below  the  sun,  or  above  it.  Occasionally,  how- 
ever, Mercury  really  passes  between  the  sun  and  the  earth. 
This  phenomenon  is  called  a  transit  of  Mercury. 

The  plane  of  the  orbit  of  Mercury,  like  that  of  the  orbit  of 
the  moon,  intersects  the  plane  of  the  ecliptic  at  two  opposite 
nodes.  The  node  at  which  the  planet  passes  north  of  the 
ecliptic  is  called  the  ascending  node,  that  at  which  it  passes 
to  the  south  of  it  the  descending  node.  It  passes  each  node 
once  in  every  revolution. 

The  line  through  the  sun  from  one  node  to  the  other  is 
called  the  line  of  nodes.  This  line  intersects  the  orbit  of  the 
earth  at  two  opposite  points,  through  one  of  which  the  earth 
passes  about  May  8  of  every  year,  and  through  the  other  about 
November  10.  Hence  when  Mercury  happens  to  pass  the 
ascending  node  within  two  or  three  days  of  November  10,  the 
sun,  planet,  and  earth  will  be  nearly  in  a  straight  line,  and  we 
see  a  transit  of  Mercury.  The  same  things  happen  when  it 
chances  to  pass  the  descending  node  within  two  or  three  days 
of  May  8.  The  following  table  shows  the  dates  of  the  trans- 
its between  1890  and  1950. 

1891,  May  9  1924,  May  7 

1894,  November  10  1927,  November  10 

1907,  November  14  1940,  November  11 
1914,  November  7 

2.  The  Planet  Venus.  Aspects  of  Venus.  — Venus  is  nearly  the 
size  of  the  earth,  its  diameter  being  only  about  300  miles  less 
than  that  of  our  globe.  It  performs  a  revolution  in  its  orbit 
around  the  sun  in  225  days,  or  about  seven  months  and  a  half. 
As  in  the  case  of  Mercury,  it  is  sometimes  in  inferior  conjunc- 
tion with  the  sun,  sometimes  at  western  elongation  ;  then  in 


154  ASTRONOMY 

superior  conjunction  and  then  in  eastern  elongation.  The  syn- 
odic period  is  1.6  years  or  about  1  y.  7  m.  3  d.  This  is  the 
interval  between  inferior  conjunctions.  Five  times  this  period 
is  8  years.  Hence  Venus  has  5  synodic  periods  in  8  years. 

At  its  greatest  elongation  Venus  is  about  45°  from  the  sun. 
It  is  then  the  most  brilliant  object  in  the  heavens  except  the 
sun  and  moon,  sometimes  casting  a  faint  but  visible  shadow. 
Hence  there  is  no  difficulty  in  recognizing  it  when  it  is  near 
either  of  its  elongations.  When  at  its  greatest  brilliancy,  it 
can  be  clearly  seen  by  a  good  eye  in  the  daytime,  provided  that 
one  knows  exactly  where  to  look  for  it. 

Morning  and  Evening  Star. — Near  greatest  eastern  elongation, 
Venus,  being  visible  in  the  west  after  sunset,  is  known  as  the 
evening  star.  The  ancients  then  called  it  Hesperus.  When 
west  of  the  sun,  it  is  seen  in  the  east  before  sunrise,  and  is 
called  the  morning  star.  The  ancients  then  called  it  Phos- 
phorus. It  is  said  that  in  the  early  ages  people  did  not  know 
that  Hesperus  and  Phosphorus  were  the  same  body.  But 
it  required  only  careful  comparison  of  observations  to  make  it 
plain  that  this  was  the  case. 

Phases  of  Venus. — To  the  naked  eye  Venus  always  appears 
like  a  star,  but  with  a  telescope  we  find  it  to  show  phases 
like  the  moon.  Figure  84  shows  these  phases.  In  the  posi- 
tion Ay  after  it  has  passed  inferior  conjunction,  most  of  its 
dark  hemisphere  is  turned  toward  us.  But  we  see  a  small 
portion  of  the  illuminated  hemisphere,  which  gives  it  the 
apparent  form  of  a  crescent,  like  that  of  the  moon  when  two 
or  three  days  old.  A  few  days  later  it  has  reached  B.  Now 
it  is  farther  from  the  earth,  so  that  it  would  look  smaller,  but 
the  crescent  is  growing  broader  because  we  can  see  a  larger 
portion  of  the  illuminated  hemisphere. 

At  C  the  planet  will  look  like  a  half  moon.  In  the  follow- 
ing positions  it  will  look  gibbous,  or  round,  and  will  appear 
smaller  and  smaller,  until  it  reaches  superior  conjunction 
The  disk  will  then  be  small  and  round.  Completing  the  revo- 
lution, the  same  phases  will  recur  in  reverse  order. 


THE  INNER  GROUP  OF  PLANETS  155 

Venus  was  one  of  the  first  objects  at  which  Galileo  pointed 
his  telescope.  He  was  greatly  delighted  when  he  found  it  to 
exhibit  phases  like  those  of  the  moon,  because  this  showed 
that  the  planet  was  an  opaque  body  revolving  round  the  sun. 


FIG.  84.  —  Showing  the  different  phases  of  Venus  during  its  synodic 
revolution. 

Supposed  Rotation  of  Venus.  —  When  viewed  with  a  good  tel- 
escope, in  a  very  steady  atmosphere,  Venus  has  a  burnished 
look,  something  like  that  of  polished  silver  shining  by  the 
light  of  a  fire.  Its  disk  seems  brighter  in  the  central  portion, 
near  the  convex  edge,  than  elsewhere.  Some  astronomers  think 
they  see  very  faint  markings,  as  shown  in  the  frontispiece.  It 
is  hence  supposed  by  some  that  the  planet  rotates  on  its  axis 
in  the  same  time  that  it  revolves  around  the  sun,  and  thus 
always  has  the  same  hemisphere  turned  to  the  sun.  Others 
have  supposed  that  it  rotates  on  its  axis  in  about  24  hours,  or 
in  nearly  the  same  time  as  the  earth.  It  is  still  uncertain 
whether  either  view  is  correct. 

Transits  of  Venus.  —  The  orbit  of  Venus  is  inclined  to  the 
ecliptic  about  3|°.  The  line  of  nodes  of  its  orbit  cuts  the 
earth's  orbit  at  points  through  which  the  earth  passes  about 
June  6  and  December  6  of  each  year.  If  Venus  happens  to 
pass  the  corresponding  node  within  a  day  or  two  of  either 
of  these  dates,  we  shall  see  her  passing  over  the  sun's 
disk.  This  phenomenon  is  called  a  Transit  of  Venus.  Such 


166  ASTRONOMY 

transits  are,  however,  among  the  rarest  phenomena  of  astron- 
omy. Two  may  occur  with  an  interval  of  eight  years  between 
them.  Then  more  than  a  century  will  elapse  before  there 
will  be  another.  Not  more  than  two  ever  occur  in  a  century, 
and  the  whole  twentieth  century  will  pass  without  any  at  all. 
The  following  table  shows  the  dates  of  all  occurring  between 
the  years  1600  and  2100 :  — 

INTERVAL 

1631,  December  7 8  years 

1639,  December  4 121 J  years 

1761,  June  5 8  years 

1769,  June  3 105$  years 

1874,  December  9       .....  8  years 

1882,  December  6 121$  years 

2004,  June  8 8  years 

2012,  June  6 

These  transits  were  formerly  looked  forward  to  with  great 
interest.  They  were  supposed  to  afford  the  best  means  of 
determining  the  distance  of  the  sun,  because  of  the  consider- 
able parallax  of  Venus  at  this  time.  Hence,  at  each  of  the  last 
four  transits,  expeditions  were  sent  to  various  parts  of  the  world 
to  make  the  most  exact  observations  possible  upon  the  times  of 
the  transit  or  the  apparent  position  of  Venus  on  the  sun's  disk. 
Such  expeditions  were  sent  out  by  the  United  States  and  other 
nations  in  1874  and  1882  to  various  points  in  Asia,  Africa  and 
Australia.  It  is  now  found,  however,  that  there  are  other 
methods  of  determining  the  sun's  distance,  more  exact  than 
this. 

3.  The  Planet  Mars.  Aspects  of  Mars.  —  Mars  is  the  fourth 
planet  in  the  order  of  distance  from  the  sun,  and  the  next  out- 
side the  orbit  of  the  earth.  Its  mean  distance  from  the  sun  is 
about  141  millions  of  miles.  The  eccentricity  of  its  orbit  is 
such  that  at  perihelion  it  is  only  128  millions  of  miles  from 
the  sun,  while  in  aphelion  it  is  154  millions  of  miles.  Hence, 
if  at  its  opposition  to  the  sun  it  should  happen  to  be  in  peri- 
helion, it  would  only  be  35  millions  of  miles  f  roin  us,  the  earth 


THE  INNER   GROUP  OF  PLANETS  157 

being  93  millions  and  Mars  128  millions  of  miles  from  the  sun. 
This  distance  is  greater  than  the  least  distance  of  Venus  from 
the  earth.  But  when  the  latter  planet  is  nearest  to  the  earth, 
its  dark  hemisphere  is  turned  toward  us,  so  we  cannot  get  a 
view  of  it.  But  when  Mars  is  nearest  to  us,  its  bright  hemi- 
sphere is  toward  us,  and  therefore  we  can  study  it  with  our 
telescopes. 

Mars  comes  into  opposition  to  the  sun  at  intervals  of  2 
years  and  1  or  2  months.  At  such  times  it  rises  near  the  time 
of  sunset,  and  may  then  be  easily  recognized  by  its  reddish 
light  and  great  brilliancy.  It  is,  indeed,  not  nearly  so  bright 
as  Venus,  but  it  is  brighter  than  any  of  the  fixed  stars.  Owing 
to  its  different  distances  from  the  sun,  it  is  much  brighter  at 
some  oppositions  than  at  others.  When  the  opposition  occurs 
in  the  month  of  August  or  September,  Mars  will  be  at  its 
brightest;  it  will  be  faintest  at  the  oppositions  occurring  in 
February  or  March. 

Surface  of  Mars.  —  When  Mars  is  examined  with  a  powerful 
telescope,  dark  and  bright  regions  are  seen  on  his  disk.  It  has 
frequently  been  supposed  that  the  bright  regions  are  conti- 
nents and  the  dark  regions  oceans.  It  was  found  by  Schiapa- 
relli  that  the  supposed  oceans  are  sometimes  joined  together  by 
dark  streaks,  which  he,  supposing  them  to  be  water,  called  chan- 
nels. Schiaparelli  was  an  Italian,  and  the  Italian  word  canale, 
which  means  channel  as  well  as  canal,  has  been  translated  canal. 
This  gave  rise  to  the  notion  of  canals  in  Mars,  which  we  fre- 
quently  read  about,  but  which  have  no  real  existence. 

Ever  since  astronomers  began  to  study  the  supposed  conti- 
nents and  oceans  on  Mars,  it  has  been  thought  that  this  planet 
has  a  surface  much  like  that  of  our  earth.  The  resemblance 
is  made  still  more  remarkable  by  the  polar  caps  of  Mars.  To 
understand  these  caps,  we  must  know  that  the  equator  of  Mars 
is  inclined  to  its  orbit  by  a  rather  greater  angle  than  the  equa- 
tor of  our  earth  is  inclined  to  the  ecliptic.  Hence,  Mars  has 
seasons  even  more  marked  than  the  earth.  During  about  one 
iialf  of  its  revolution  its  north  pole  is  turned  away  from  the 


158  ASTRONOMY 

sun.  During  this  time  it  is  found  that  a  bright,  white  cap  is 
formed  around  the  pole.  When  the  sun  begins  to  shine  on  the 
pole  this  cap  gradually  melts  away,  or  at  least  becomes  much 


FIG.  86.  — Four  drawings  of  Mars  made  by  Barnard  with  the  great  Lick 

Telescope. 

smaller.  The  same  thing  takes  place  round  the  south  pole  of 
the  planet  during  the  other  half  of  the  revolution.  It  is  there- 
fore supposed  that  these  caps  consist  of  snow  or  frost  which 


THE  INNER   GROUP   OF  PLANETS  159 

falls  or  is  deposited  during  the  Martian  winter,  and  melts 
away  again  during  the  Martian  summer. 

The  atmosphere  of  Mars  is  very  rare ;  indeed,  it  is  not  cer- 
tain that  this  planet  has  any  atmosphere  at  all  that  we  can  get 
evidence  of. 

Rotation Mars  revolves  on  its  axis  in  24  h.  37  m.  Hence 

its  day  is  a  little  longer  than  ours  is. 

Supposed  Inhabitants.  —  It  is  generally  supposed  that  Mars 
may  be  inhabited.  This,  however,  is  purely  a  supposition, 
because  we  can,  with  our  telescopes,  get  no  evidence  on  one  side 
or  the  other  of  the  question.  The  only  reason  we  have  to  believe 
in  such  inhabitants  is  the  fact  that  our  earth  is  inhabited. 

The  Satellites  of  Mars —  These  satellites  were  discovered  by 
Professor  Hall  in  August,  1877.  Previous  to  that  time  it  was 
supposed  that  Mars  had  no  satellites.  These  bodies  are  the 
smallest  yet  known  in  the  solar  system,  and  can  be  .seen  only 


FIG.  86.  —Apparent  orbits  of  the  satellites  of  Mars  in  1877. 

with  powerful  telescopes,  and  under  favorable  conditions.  The 
inner  one  moves  round  its  primary  in  a  shorter  time  than  any 
other  known  satellite,  making  a  revolution  in  7  h.  38  m.  This 
is  less  than  one  third  the  day  of  Mars,  consequently  to  aft 
inhabitant  of  that  planet  Phobos  rises  in  the  west  and  sets  in 
the  east.  Its  distance  from  the  surface  of  the  planet  is  only 
about  4000  miles,  and  therefore  a  little  more  than  half  the 
diameter  of  the  earth.  Our  moon  is  about  sixty  times  this 
distance  from  the  earth. 


160  ASTRONOMY 

4.  The  Minor  Planets  or  Asteroids Four  of  these  planets 

were  discovered  during  the  early  part  of  the  nineteenth  cen- 
tury, between  the  years  1801  and  1807.  Then  no  more  were 
found  until  1845.  In  that  year  a  fifth  was  discovered,  soon 
after  a  sixth,  and  then  they  began  to  be  found  in  great  num- 
bers, sometimes  12  or  more  in  a  single  year.  The  number 
known  is  now  approximating  500,  and  new  ones  are  found  so 
frequently  that  we  do  not  know  what  the  total  number  may  be. 

In  recent  years  discoveries  of  these  bodies  have  mostly  been 
made  by  photography.  To  apply  this  method,  photographs  of 
the  sky,  the  plate  being  exposed  for  several  hours,  are  taken. 
The  stars  appear  on  the  plates  as  points.  But  if  a  planet  is 
photographed  it  will  appear  as  a  little  line  in  consequence  of 
its  motion.  In  this  way  a  new  minor  planet  is  found  from 
time  to  time. 

These  bodies  are  far  smaller  than  the  major  planets.  Their 
size  is  not  exactly  known,  but  the  largest  are  probably  about 
400  or  500  miles  in  diameter ;  the  smallest  not  more  than  20. 
The  diameters  of  the  smaller  ones  cannot  be  given  with  cer- 
tainty, because  they  are  so  small  that  the  planet  appears  only 
as  a  point  of  light  in  the  largest  telescope. 

The  eccentricities  of  the  orbits  of  the  minor  planets  are 
generally  larger  than  those  of  other  planets,  and  their  orbits 
are  frequently  more  inclined  to  the  ecliptic.  They  are  there- 
fore scattered  widely  through  the  region  which  they  occupy. 
Yet  it  is  probable  that  if  they  were  all  combined  into  a  single 
planet,  the  mass  of  this  planet  would  be  smaller  than  that  of 
any  major  planet,  even  Mercury. 

Olbers's  Hypothesis.  —  When  only  four  of  these  planets  were 
known,  it  was  supposed  that  they  were  fragments  of  a  large 
planet  which  had,  in  some  way,  been  broken  to  pieces.  This 
idea  was  first  propounded  by  the  astronomer  Olbers,  hence  it 
has  since  borne  his  name.  But  it  is  now  found  that  this  could 
not  have  been  the  case,  because  the  orbits  cover  too  much 
space,  and  have  never  intersected  each  other,  as  they  would 
have  done  had  such  a  breaking  up  occurred. 


THE  IN  NEE   GROUP  OF  PLANETS  161 

The  Planet  Eros.  —  The  most  remarkable  of  these  planets  is 
one  discovered  in  1898,  and  called  Eros.  Its  orbit,  instead  of 
being  wholly  outside  that  of  Mars,  is  partly  outside  and  partly 
inside.  If  it  were  not  for  the  great  inclination  of  the  orbit  of 
Eros,  it  would  intersect  the  orbit  of  Mars.  But  it  is  inclined 
so  much  that  it  passes  above  the  orbit  of  Mars  at  one  point, 
and  below  it  at  another.  In  fact,  the  two  orbits  are  linked 
together  in  such  a  way  that,  if  each  were  made  of  wire,  they 
would  pass  through  each  other  like  two  links  of  a  chain. 

Eros,  at  certain  times,  comes  nearer  the  earth  than  any 
other  body  of  the  solar  system,  the  moon  excepted.  Hence, 
observations  upon  it  may  enable  its  distance  to  be  measured 
with  greater  exactness  than  can  be  attained  in  the  case  of  any 
other  planet. 


NEWCOMB'S  ASTRON. — 11 


CHAPTER  XII 
THE  FOUR  OUTER   PLANETS 

1.  The  Planet  Jupiter.  —  Jupiter  is  sometimes  called  the 
giant  planet.  Not  only  is  he  the  largest  and  most  massive 
of  the  planets,  but  his  mass  and  size  are  greater  than  those  of 
all  the  other  planets  combined.  His  mean  diameter  is  about 
85,000  miles,  and  his  volume  is  therefore  1300  times  that  of 
our  earth. 

Jupiter  rotates  on  his  axis  in  the  remarkably  short 
period  of  9  h.,  55  m.  The  centrifugal  force  generated  by 
this  rapid  rotation  makes  him  assume  a  very  oblate  figure. 
His  equatorial  diameter  exceeds  his  polar  diameter  by  5000 
miles.  The  ellipticity  of  his  disk  is  therefore  plainly  visible 
with  a  telescope. 

A  very  remarkable  feature  of  the  material  composing  this 
planet  is  that  its  specific  gravity  is  scarcely  one  fourth  that 
of  the  earth,  and  only  about  one  third  greater  than  that  of 
water,  hence,  although  1300  times  the  volume  of  the  earth,  the 
planet  has  only  313  times  the  mass  of  the  earth. 

Jupiter  revolves  around  the  sun  in  nearly  12  years.  He 
comes  into  opposition  to  the  sun  at  intervals  of  about  13 
months.  When  in  opposition  he  rises  at  sunset  and  may  be 
well  seen  in  the  evening.  In  brightness  he  is,  when  seen  by 
the  naked  eye,  between  Sirius  and  Venus.  He  can  be  dis- 
tinguished from  Mars  by  his  whiter  light,  which  is  very  differ- 
ent from  the  reddish  light  of  Mars. 

Surface  of  Jupiter.  —  The  most  remarkable  features  of  the 
surface  of  Jupiter  consist  of  certain  dark,  cloudlike  bands, 
two  of  which,  north  and  south  of  his  equator,  are  especially 


THE  FOUR   OUTER  PLANETS 


163 


marked.    These  bands  can  be  seen  with  a  very  small  telescope, 
and  have  been  known  for  more  than  two  centuries. 


of  Uranus 


FIG.  87.  —  Orbits  of  the  superior  planets. 

When  examined  with  a  powerful  telescope  it  is  seen  that 
not  only  these  bands,  but  the  entire  surface  of  the  planet,  are 
variegated  with  shadings,  differing  greatly  in  form  and  bril- 
liancy. The  bands  especially  consist  of  great  numbers  of 


164  ASTRONOMY 

stratified,   cloudlike  appearances.      Sometimes    small    bright 
spots  are  seen  scattered  here  and  there  over  the  disk. 

Generally  the  details  on  the  surface  of  Jupiter  change  from 
day  to  day,  so  that  no  permanent  map  of  the  planet  can  be 
made.  If  an  exact  drawing  of  the  planet  is  made  on  one 
evening,  it  will  be  found,  two  or  three  evenings  later,  when 
the  same  side  is  presented  to  us,  that  many  changes  have 
taken  place.  Sometimes,  however,  spots  are  seen  which 
remain  for  weeks,  months,  or  even  years.  Most  remarkable 


Fio.  88.  —  Telescopic  view  of  Jupiter  with  the  dark  shadow  of  a  satellite 
crossing  over  its  disk. 

of  these  was  a  red  spot  which  appeared  south  of  the  equator 
in  the  year  1878  or  1879.  It  varied  somewhat  from  time  to 
time,  but  continued  without  any  material  diminution  for  more 
than  ten  years.  Then  it  began  to  fade  out  in  a  varying  and 
uncertain  way,  being  sometimes  conspicuous  and  sometimes 
seen  with  difficulty.  Since  1892  it  has  been  only  occasionally 
visible. 

From  the  variability  of  the  surface,  we  conclude  that  what 
we  see  on  Jupiter  is  not  land,  and  probably  not  water.     When 


THE  FOUR  OUTER  PLANETS  165 

carefully  examined,  the  edge  of  the  disk  is  not  sharply  de- 
nned, but  appears  much  softened,  the  illumination  there  being 
far  less  than  it  is  at  the  surface.  All  this  leads  us  to  sur- 
mise that  what  we  see  on  the  surface  of  Jupiter  are  great 
banks  of  clouds  floating  in  an  atmosphere,  and  carried  about 
by  strong  winds  blowing  mostly  in  an  east  and  west  direction. 
These  clouds  are  so  thick  that  we  cannot  see  the  body  of  the 
planet  through  them.  Hence  it  is  not  known  whether  this 
body  is  solid  or  liquid.  It  is  sometimes  supposed  to  be  like 
the  sun,  an  extremely  hot  mass,  liquid  or  vaporous,  com- 
pressed by  the  weight  of  its  outer  portions. 

2.  The  Satellites  of  Jupiter.  —  When  we  look  at  Jupiter 
through  a  telescope,  or  even  through  a  good  spyglass,  we  shall 
generally  see  three  or  four  bright  objects  near  him.  These 
are  his  satellites,  which  were  discovered  by  Galileo,  and  be- 
came at  once  objects  of  the  greatest  interest  to  astronomers. 
Galileo  saw  that  these  objects  revolved  around  Jupiter  as  the 
moon  revolves  around  the  earth,  and  the  planets  around  the 
sun.  At  that  time  the  doctrine  that  the  earth  and  planets 
revolved  around  the  sun  was  not  generally  held.  But  Galileo 
believed  in  it,  and  saw  in  the  revolutions  of  the  satellites  an 
additional  strong  proof,  by  analogy,  of  the  revolution  of  the 
planets.  It  is  said  that  one  astronomer  thought  the  satellites 
were  illusions  produced  by  the  telescope  itself.  There  is  also 
a  story  of  a  philosopher  who  refused  to  put  his  eye  to  the 
telescope  lest  he  should  see  the  satellites,  and  be  convinced. 
He  died  shortly  afterward.  "  I  hope,"  said  Galileo,  "  that  he 
saw  them  on  his  way  to  heaven." 

It  has  sometimes  been  a  question  whether  these  bodies  can- 
not be  seen  without  a  telescope.  It  is  certain  that  if  Jupiter 
were  out  of  the  way  they  would  be  visible  to  the  naked  eye. 
It  seems  likely  that,  when  two  of  them  happened  to  be  close 
together,  they  have  been  seen  as  a  single  satellite,  in  spite  of 
the  glare  of  Jupiter. 

A  fifth  satellite,  nearer  to  the  planet  than  the  other  four, 


166  ASTRONOMY 

was  discovered  by  Barnard  at  the  Lick  Observatory  of  Cali- 
fornia in  1892.  It  is,  so  far  as  known,  the  faintest  and  most 
difficult  satellite  to  see  in  the  solar  system,  being  visible  only 
in  the  most  powerful  telescopes.  Even  with  such  a  telescope 
a  well-trained  eye  is  necessary.  The  four  largest  satellites 
are  probably  about  the  size  of  our  moon,  or  a  little  larger. 
They  are  so  much  fainter  than  the  moon,  not  only  on  account 
of  their  greater  distance  from  us,  but  because  of  their  fainter 
illumination  by  the  sun,  being  five  times  farther  away  from 
the  latter  than  we  are. 

Eclipses  and  Transits  of  the  Satellites.  —  It  is  evident  that 
a  planet,  being  an  opaque  body  illuminated  by  the  sun,  must 
cast  a  shadow,  as  our  earth  does.  Whenever  a  satellite  enters 
this  shadow  it  undergoes  an  eclipse  like  one  of  our  moon.  In 
the  case  of  Jupiter's  satellites  these  eclipses  can  be  easily 
observed.  The  inclination  of  their  orbits  to  that  of  the  planet 
is  so  small,  and  the  planet  itself  is  so  large,  that  all  the 
satellites  except  the  outer  one  pass  through  the  shadow  of  the 
planet  at  every  revolution.  Accordingly,  if  we  view  one  of 
these  bodies  when  it  is  going  to  pass  behind  the  planet,  we 
shall  often  see  that,  before  it  reaches  the  planet,  it  fades  out 
and  disappears  from  view.  This  is  because  it  is  entering  the 
shadow.  At  other  times  it  can  be  seen  coming  into  view  as 
it  emerges  from  the  shadow. 

These  eclipses  are  interesting  subjects  of  observation,  as 
they  can  be  seen  with  quite  a  small  telescope.  Their  times 
are  predicted  in  astronomical  ephemerides,  so  that  any  observer 
with  such  a  publication  can  observe  these  eclipses  very  fre- 
quently if  the  planet  is  not  too  near  the  sun. 

Frequently,  also,  the  satellites  pass  between  us  and  the 
planet  Jupiter.  This  is  a  phenomenon  which  it  is  very  inter- 
esting to  watch.  When  the  satellite  first  enters  upon  the  disk, 
it  commonly  appears  bright  on  the  darker  background  of  the 
planet ;  but,  as  it  approaches  the  center  of  the  disk,  it  frequently 
looks  darker  than  the  disk.  This  is  because  the  center  of  the 
disk  of  Jupiter  is  brighter  than  the  edge. 


THE  FOUR   OUTER  PLANETS  167 

When  a  satellite  is  thus  passing  across  the  disk,  we  can 
frequently  see  its  shadow  traveling  over  the  disk  near  it.  This 
is  also  a  very  interesting  phenomenon  to  watch,  but  for  this 
purpose  we  need  a  good-sized  telescope.  (See  fig.  88.) 

3.  The  Planet  Saturn.  —  Among  the  planets  Saturn  is  next 
to  Jupiter  in  size,  its  mass  being  about  one  third  that  of  the 
giant  planet.  It  revolves  around  the  sun  in  29J  years.  Al- 
though smaller  than  Jupiter,  it  has  about  three  times  the  mass 
of  the  six  smaller  planets  put  together. 

Saturn  has  many  points  of  resemblance  to  Jupiter.  These 
are:  — 

1.  Its  rapid  rotation  on  its  axis.     The  time  of  rotation  is 
10  h.  14  m.,  less  than  20  minutes  greater  than  that  of  Jupiter. 

2.  Its  small  density,  which  is  even  less  than  that  of  water, 
and,  so  far  as  we  yet  know,  less  than  that  of  any  other  planet 
or  satellite. 

3.  The  cloudlike  aspect  of  its  surface.     But  these  cloud 
forms  are  far  less  distinct  than  they  are  on  Jupiter,  and  so  can 
be  recognized  only  with  difficulty. 

4.  The  number  of  its  satellites,  it  having  eight  or  nine  in  all, 
three  or  four  more  than  Jupiter. 

Saturn  is  redder  in  color  than  Jupiter  and  not  so  bright. 
The  star  which  it  most  resembles  in  appearance  to  the  naked 
eye  is  Arcturus. 

4.  The  Rings  of  Saturn.  —  When  seen  with  a  large  telescope, 
Saturn  is  the  most  wonderful  object  in  the  solar  system,  owing 
to  the  magnificent  rings  which  surround  it.  The  appearance 
of  these  rings  is  shown  in  the  picture  of  the  planet.  They  are 
perfectly  flat,  and  so  thin  that,  when  seen  edgeways,  they 
nearly  or  quite  disappear,  even  in  powerful  telescopes. 

They  appear  to  be  two  in  number,  separated  by  a  very  nar- 
row dark  line,  as  shown  in  the  figure.  The  outer  ring,  it  will 
be  noticed,  is  much  narrower  than  the  inner  one.  Near  the 
two  ends,  which  are  called  its  ansce,  a  little  dark  shading  is 


168  ASTRONOMY 

seen  on  it.  It  is  not  yet  certain  whether  this  shading  indicates 
a  division  of  this  ring  into  two  others,  or  whether  it  arises 
from  this  part  of  the  ring  being  composed  of  darker  matter 
than  the  remainder. 

The  inner  ring  is  brightest  near  its  outer  edge,  and  grows 
darker  toward  the  planet.  Its  inner  portion  is  so  dark  as  not 
to  be  visible  except  in  a  pretty  large  telescope.  When  this 
portion  was  first  noticed,  it  was  thought  to  be  a  separate  ring, 


FIG.  89.  — Telescopic  view  of  Saturn  and  its  rings. 

and  was  therefore  called  the  crape  ring  or  dusky  ring.  It  is 
now,  however,  found  to  be  joined  continuously  to  the  rest  of 
the  inner  ring. 

The  rings  are  inclined  to  the  plane  of  the  planet's  orbit  by 
about  28°.  The  direction  of  their  plane  remains  nearly  un- 
changed as  the  planet  revolves  around  the  sun.  Hence,  in 
some  positions  in  the  orbit,  we  see  the  rings  considerably 


THE  FOUR   OUTER  PLANETS  169 

inclined,  as  shown  in  the  figure.  As  the  planet  moves  around, 
the  rings  appear  to  grow  narrower,  through  the  greater  obliquity 
at  which  we  see  them.  At  length  they  close  down  into  a  mere 
line,  and  may  quite  disappear.  As  the  planet  goes  on,  we 
see  the  other  side  of  the  rings  and  they  gradually  open  out 


FIG.  90.  —  Drawings  of  Saturn  and  its  rings  made  by  various  astronomers 
between  1610  and  1656,  before  they  could  see  with  their  poor  teles- 
copes what  the  rings  were. 

again.     The  disappearance  occurs  at  every  half  revolution,  or 
about  once  in  fifteen  years. 

When  these  rings  were  first  seen,  they  were  a  great  puzzle 
to  the  astronomical  observers,  who  could  not  distinguish  their 


170  ASTRONOMY 

true  shape.  To  Galileo  they  seemed  like  two  little  handles  to 
the  planet,  because  he  could  see  only  the  parts  which  pro- 
jected. After  two  or  three  years  Saturn  moved  in  such  a 
position  that  the  rings  were  seen  edgewise,  then  they  disap- 
peared entirely  from  Galileo's  view.  It  is  said  that  the  philos- 
opher was  greatly  perplexed  by  this,  not  knowing  how  a  celes- 
tial body  could  vary  in  this  way,  and  feared  that  something 
might  be  wrong  in  his  observations.  We  give  a  figure  (page 
169),  showing  some  of  the  drawings  made  by  observers  during 
the  years  between  1610  to  1656.  In  1656  the  true  form  of  the 
object  was  made  known  by  the  celebrated  Huyghens. 

The  question  of  the  constitution  of  the  rings  was  long  a 
difficult  one  because  it  was  impossible  to  conceive  how  immense 
solid  bodies,  as  they  were  supposed  to  be,  could  be  sustained 
without  falling  on  the  planet.  But  it  is  now  known  that  they 
are  not  solid  bodies  at  all,  but  only  clouds  of  small  particles, 
or  rings  of  dust  or  vapor.  These  little  objects  are  thousands 
and  perhaps  millions  in  number,  each  revolving  in  its  own 
orbit.  They  seem  to  us  to  form  a  continuous  surface,  owing  to 
their  great  number. 

5.  The  Satellites  of  Saturn.  —  Saturn  has  eight  satellites, 
(perhaps  nine)  revolving  around  it,  a  larger  number  than  any 
other  planet.  The  brightest  of  these  was  discovered  by 
Huyghens,  in  1655.  A  few  years  later,  Cassini,  of  Paris,  dis- 
covered a  second  one.  As  the  telescope  was  improved,  new 
ones,  nearer  to  the  planet,  were  added.  The  nearest  one  of  all, 
which  revolves  near  the  outer  edge  of  the  ring,  was  discovered 
by  Sir  William  Herschel.  The  faintest  and  most  difficult  of 
the  eight  to  see,  called  Hyperion,  was  discovered  by  Bond,  at 
Cambridge,  in  1848.  A  ninth  satellite,  more  distant  than  any 
of  the  others,  was  thought  to  be  photographed  by  W.  H. 
Pickering  at  the  Harvard  Observatory,  in  1899,  but  its  peri- 
odic time  is  not  ascertained.  The  following  table  gives  the 
names  of  the  satellites,  their  distances  from  the  planet  in  radii 
of  the  planet,  their  discoverer,  and  the  date  of  discovery ;  — 


THE  FOUR   OUTER  PLANETS 


171 


No. 

Name 

Distance 
from  Saturn 

Periodic 
Time 

Discoverer  and  Date 

1 

Mimas 

3.3 

Od.  23  h. 

Herschel  in  1789 

2 

Enceladus 

4.3 

Id.    9h. 

Herschel  in  1789 

3 

Tethys 

5.3 

1  d.  21  h. 

Cassini  in  1684 

4 

Dione 

6.8 

2  d.  18  h. 

Cassini  in  1684 

5 

Rhea 

9.5 

4d.  12  h. 

Cassini  in  1672 

6 

Titan 

20.7 

16  d.  23  h. 

Huyghens  in  1655 

7 

Hyperion 

26.8 

21  d.    7  h. 

Bond  in  1848 

8 

Japetus 

64.4 

79  d.  22  h. 

Cassini  in  1671 

There  seem  to  be  two  gaps  in  the  distances,  one  between 
Rhea  and  Titan,  the  other  between  Hyperion  and  Japetus. 
We  might  suppose  that  there  are  little  satellites  in  these  gaps, 
but,  if  so,  the  most  careful  search  with  the  most  powerful 
telescopes  has  failed  to  reveal  them. 

These  objects  are  of  very  unequal  brightness.  The  innermost 
one,  Mimas,  and  the  seventh,  Hyperion,  can  be  seen  only  with 
large  telescopes.  Enceladus  is  nearly  as  difficult  as  Mimas. 
Titan  can  be  seen  with  a  very  small  telescope.  The  others 
are  intermediate  in  difficulty. 

All  these  satellites,  except  the  outer  one,  revolve  in  the  plane 
of  the  ring.  Consequently  when  the  edge  of  the  ring  is  turned 
toward  the  earth,  the  satellites  seem  to  swing  from  one  side  of  the 
planet  to  the  other  in  a  straight  line,  running  along  the  ring  like 
beads  on  a  string.  The  planes  of  the  orbits  are  kept  together 
by  the  attraction  of  the  rings  and  satellites  on  each  other. 

Japetus,  the  outer  satellite,  has  a  remarkable  peculiarity. 
It  is  much  brighter  when  seen  west  of  the  planet  than  when 
seen  east  of  it.  The  explanation  of  this  is  that  the  satellite, 
like  our  moon,  always  presents  the  same  face  to  the  planet, 
and  that  one  hemisphere  is  much  whiter  in  color  than  the  other. 
The  result  is  that  the  white  hemisphere  is  turned  toward  us 
when  it  is  on  one  side  of  the  planet,  and  the  dark  one  when  it 
is  on  the  other  side. 


172  ASTRONOMY 

6.  Uranus  and  its  Satellites.  —  The  distance  of  Uranus  from 
the  sun  is  about  twice  that  of  Saturn.  When  near  opposition 
it  shines  as  a  star  of  the  sixth  magnitude.  It  can  therefore 
be  seen  with  the  naked  eye  if  one  knows  exactly  where  to  look 
for  it. 

In  a  small  telescope  this  planet  looks  like  a  star.  But  when 
a  magnifying  power  of  several  hundred  is  used,  it  will  be  seen 
to  have  a  disk.  It  has  a  greenish  tint,  and  the  spectroscope 
shows  that  it  is  surrounded  by  a  very  dense  atmosphere.  Its 
time  of  rotation  on  its  axis  is  unknown. 

Uranus  was  discovered  by  Sir  William  Herschel  in  1781. 
He  at  first  supposed  it  to  be  a  comet.  After  a  few  weeks  its 
motion  showed  that  this  could  not  be  the  case,  and  its  true 
nature  was  then  soon  learned.  Herschel  proposed  to  call  it 
the  Georgium  Sidus,  or  the  Georgian  Star,  after  King  George, 
who  had  afforded  him  the  means  of  making  his  discoveries. 
This  name  was  not  used  outside  of  England.  Some  astrono- 
mers proposed  to  call  the  planet  Herschel,  after  its  discoverer, 
but  this  name  did  not  meet  with  favor  either.  The  name 
Uranus  was  then  chosen  and  permanently  adopted. 

Satellites  of  Uranus. —  A  few  years  after  the  discovery  of  the 
planet,  Herschel  found  that  it  was  accompanied  by  two 
satellites.  Afterward,  he  thought  he  found  four  others,  mak- 
ing six  in  all.  It  was  therefore  supposed  for  a  long  time  that 
Uranus  had  six  satellites.  But  recent  investigations  with  the 
more  powerful  telescopes  of  our  time  have  shown  that  the 
four  smaller  supposed  satellites  did  not  belong  to  Uranus,  but 
were  probably  fixed  stars  which  Herschel  had  from  time  to 
time  seen  in  the  neighborhood  of  the  planet.  But  two  addi- 
tional very  faint  satellites  were  discovered  by  Lassell  quite 
near  the  planet.  The  existence  of  these  has  been  well  authen- 
ticated by  recent  observations.  There  is  probably  no  great 
difference  in  the  actual  brightness  of  the  four  satellites ;  but  it 
requires  a  powerful  telescope  to  see  any  of  them,  and  those 
nearer  to  the  planet  are  harder  to  see  than  the  two  distant 
ones,  because  of  the  glare  of  the  planet. 


THE  FOUR   OUTER  PLANETS  173 

These  satellites  have  one  remarkable  peculiarity.  The 
orbits  of  the  satellites  of  the  earth,  Mars,  Jupiter,  and  Saturn 
are  but  little  inclined  to  the  plane  of  the  ecliptic ;  but  those  of 
Uranus  are  nearly  perpendicular  to  this  plane.  That  is  to  say, 
the  plane  in  which  they  revolve  is  not  slightly  inclined,  like 
the  other  planes  we  have  described,  but  is  tipped  up  at  almost 
a  right  angle.  Consequently,  when  the  planet  is  in  certain 
parts  of  its  orbit,  we  see  these  orbits  almost  perpendicularly, 
and  can  watch  the  satellites  in  their  whole  course  around  the 
planet.  As  Uranus  revolves  in  its  orbit,  the  plane  of  the 
orbits  of  the  satellites  retains  its  direction.  Consequently, 
twice  in  every  revolution  of  Uranus  this  plane  passes  through 
the  position  of  the  earth.  The  motions  of  the  satellites  then 
seem  to  take  place  in  a  nearly  north  and  south  direction, 
swinging  on  each  side  of  the  planet. 

7.  Neptune  and  its  Satellite.  —  Neptune  is,  so  far  as  we 
know,  the  outermost  planet  of  our  system.  Its  discovery  was 
one  of  the  most  remarkable  events  in  the  history  of  astronomy, 
illustrating  the  great  exactness  of  astronomical  theory  and  the 
power  of  the  human  intellect  in  penetrating  the  mysteries  of 
the  celestial  motions.  Its  existence  was  predicted  and  its 
direction  from  the  earth  made  known  before  it  had  been  recog- 
nized by  the  human  eye. 

About  forty  years  after  the  discovery  of  Uranus,  tables  for 
calculating  the  motions  of  that  planet  were  made  by  Bouvard 
of  Paris.  Such  tables  enable  us  to  calculate  not  only  how  the 
planet  would  move  in  an  elliptic  orbit  under  the  influence  of 
the  sun's  attraction,  but  also  to  what  extent  and  in  what  direc- 
tion it  is  drawn  away  from  this  elliptic  orbit  by  the  attractions 
of  the  other  planets.  Soon  after  Bouvard's  tables  were  pub- 
lished, it  was  found  that  the  planet  did  not  move  exactly  in 
accordance  with  them.  The  deviation  was  indeed  very  small, 
even  when  at  its  greatest.  How  small  it  was,  we  may  judge 
by  the  fact  that,  if  there  had  been  two  planets  in  the  heavens, 
the  one  in  the  position  of  the  real  Uranus,  and  the  other  in  the 
position  where  Bouvard's  tables  said  the  planet  ought  to  be, 


174  ASTRONOMY 

the  two  would  have  seemed  to  the  naked  eye  as  a  single  star. 
But  in  a  telescope  this  small  difference  was  very  perceptible, 
and  the  deviations  were  a  source  of  surprise  to  observers. 

About  1843  it  occurred  to  several  astronomers  that  these 
deviations  were  probably  due  to  the  planet  being  drawn  from 
its  place  by  the  attraction  of  some  unknown  planet,  probably 
one  whose  orbit  lay  outside  that  of  Uranus.  Two  mathema- 
ticians, Leverrier  in  Paris  and  Adams  in  England,  thereupon 
undertook  to  calculate  where  this  unknown  planet  should  be 
situated  and  how  it  should  move  in  order  that  its  attraction 
might  produce  the  observed  motions  of  Uranus.  The  two  men 
agreed  remarkably  in  their  conclusions.  The  difference  in  the 
directions  which  they  assigned  to  the  unknown  planet  was  only 
one  or  two  degrees. 

Leverrier  knew  that  the  astronomers  of  Berlin  were  engaged 
in  making  maps  of  the  stars  in  that  part  of  the  sky  where  the 
planet  should  be.  It  occurred  to  him  that  it  might  be  found 
by  the  aid  of  such  a  map.  He  therefore  wrote  to  Doctor  Galle 
in  the  autumn  of  1846,  asking  him  to  examine  the  map  and 
compare  it  with  the  heavens.  Pointing  his  telescope  in  the 
required  direction,  Galle  found  an  object  which  was  not  on  his 
map  of  the  stars.  But  he  could  not  tell  certainly  whether  it 
was  a  star  until  the  following  evening.  Then  he  looked  again 
and  found  that  the  object  had  changed  its  position.  This 
showed  that  it  was  really  a  planet ;  and  it  proved  to  be  the 
one  which  had  been  predicted.  This  result  was  of  the  great- 
est interest,  and  most  of  the  astronomers  of  the  world  who  had 
instruments  at  their  disposal  began  to  watch  the  course  of  the 
newly  discovered  body. 

Satellite  of  Neptune. — Mr.  Lassell  of  England,  soon  after 
the  discovery  of  Neptune,  noticed  a  very  faint  star  near  it. 
After  watching  for  a  few  evenings,  this  star  proved  to  be  a 
satellite.  It  is  the  only  one  that  Neptune  is  known  to  have. 
Its  orbit  is  inclined  to  the  ecliptic  about  30°.  But  its  motion 
has  the  remarkable  peculiarity  of  being  retrograde  instead  of 
direct.  That  is  to  say,  if  we  should  look  down  upon  the  solar 


THE  FOUR   OUTER  PLANETS  175 

system  from  a  great  height  in  a  direction  perpendicular  to  the 
plane  of  the  ecliptic,  we  should  see  all  the  planets  and  satel- 
lites making  their  revolutions  in  the  opposite  direction  from 
that  of  the  hands  of  a  clock,  the  satellites  of  Uranus  and  Nep- 
tune excepted.  Those  of  Uranus,  as  I  have  just  explained, 
would  appear  to  move  nearly  in  the  same  plane  in  which  we 
were  looking  down  on  them,  swinging  back  and  forth  on  each 
side  of  the  planet,  but  that  of  Neptune  would  move  in  the 
opposite  direction  from  all  the  others. 

The  orbit  of  this  satellite  changes  its  position  in  such  a  way 
as  to  show  that  the  planet  is  spheroidal  in  form.  It  is  there- 
fore concluded  that  it  has  a  rapid  rotation  about  its  axis.  But 
it  is  not  possible  to  detect  this  rotation  with  a  telescope, 
owing  to  the  great  distance  of  the  planet. 


CHAPTER  XIII 
COMETS  AND  METEORS 

1.  Appearance  of  a  Comet.  —  Comets  are  objects  of  unusual 
aspect  which,  to  the  ordinary  observer,  sometimes  seem  to 
hover  in  the  heavens  for  a  few  weeks  or  months,  and  then 
gradually  disappear.  Occasionally  one  of  these  objects  is  so 
large  and  brilliant  as  to  command  universal  attention.  Smaller 
ones,  that  would  hardly  be  noticed  unless  the  attention  were 
called  to  them,  are  more  frequent.  Smaller  ones  yet,  that  can- 
not be  seen  except  with  a  telescope,  appear  in  such  numbers 
that  a  year  seldom  passes  without  several  being  observed. 

Parts  of  a  Comet.  —  A  large  comet  consists  of  three  parts, 
the  nucleus,  the  coma,  and  the  tail.  These  parts  are  not  com- 
pletely distinct,  as  one  merges  gradually  into  the  other. 

The  nucleus,  to  the  naked  eye,  looks  like  a  star.  It  would 
not  excite  notice  but  for  the  coma  and  tail  by  which  it  is 
accompanied. 

The  coma  (Latin  for  hair)  is  a  mass  of  cloudy  or  vaporous 
appearance  in  which  the  nucleus  is  enveloped  and  which 
shades  off  so  gradually  that  we  cannot  say  precisely  where  it 
ends.  It  gives  the  nucleus  the  appearance  of  a  star  shining 
through  a  little  bunch  of  fog.  The  nucleus  and  coma  together 
are  called  the  head  of  the  comet. 

The  tail  is  a  continuation  of  the  coma,  and  consists  of  a 
stream  of  foggy  or  milky  light,  growing  broader  and  fainter  as 
it  recedes  from  the  head,  until  the  eye  can  no  longer  trace  it. 

The  direction  of  the  tail  is  always  away  from  the  sun.  Its 
extent  is  very  different  in  different  comets.  In  some  of  the 
largest  it  extends  over  a  considerable  arc  of  the  heavens,  while 

176 


COMETS  AND  METEORS  177 

in  others  it  is  comparatively  short.  Its  actual  length  is  nearly 
always  many  millions  of  miles. 

Variety  of  Aspects  of  a  Comet.  —  Comets  that  cannot  be  seen 
by  the  naked  eye  are  called  telescopic  comets.  These  objects 
frequently  have  no  tail,  and  occasionally  the  nucleus  is  so 
faint  as  to  be  scarcely  visible  even  in  the  telescope.  In  these 
cases  the  comet  looks  like  a  minute  patch  of  fog. 

A  periodic  comet  is  one  which  is  known  to  perform  a  regular 
revolution  round  the  sun,  like  a  planet.  These  comets  move 
in  much  more  eccentric  orbits  than  planets. 

Most  of  the  periodic  comets  make  their  revolution  in  a  few 
years,  between  three  years  and  fifteen.  But  there  are  also  a 
few  comets  having  periods  ranging  from  seventy  years  upward. 
Only  two  or  three  of  these  have  ever  been  seen  at  more  than 
one  return. 

2.  Comets  belong  to  the  Solar  System.  —  It  is  now  known  that 
all  comets  may  be  considered  as  belonging  to  the  solar  system. 
They  are  attracted  by  the  sun  as  the  planets  are ;  but  instead 
of  moving  in  nearly  circular  orbits,  like  the  planets,  they  drop 
down,  as  it  were,  from  an  immense  distance,  generally  from  a 
region  far  outside  the  orbit  of  Neptune.  If  one  fell  exactly 
toward  the  sun,  it  would  drop  into  it.  But  every  comet 
hitherto  known  has  its  motion  directed  a  little  one  side  of  the 
sun  or  the  other.  As  it  drops  it  goes  faster  and  faster  until, 
when  it  gets  very  near  the  sun,  it  is  moving  with  such  rapidity 
that  the  sun  can  no  longer  hold  it.  It  whirls  around  and  goes 
off  again,  nearly  in  the  direction  from  which  it  came.  The 
ellipse  in  which  it  is  supposed  to  move  is  so  elongated  that  we 
cannot  see  the  comet  in  any  part  of  it  except  quite  near 
the  sun. 

Comets  are  entirely  invisible  except  in  that  small  part  of 
their  orbit  nearest  to  the  earth  and  sun.  As  one  is  dropping 
toward  the  sun,  some  keen-eyed  astronomer,  with  a  little  tele- 
scope, is  almost  sure  to  detect  it  if  the  earth  is  in  a  favorable 
position  for  seeing  it.  He  then  announces,  his  discovery  by 
NEWCOMB'S  ASTRON.  — 12 


178 


ASTRONOMY 


telegraph  to  his  fellow  astronomers  in  Europe  and  America, 
telling  exactly  where  it  is  to  be  seen.  When  first  found  it  com- 
monly looks  like  a  mere  patch  of  fog.  Then  as  it  gets  nearer 
and  brighter  the  nucleus  is  seen,  and  if  the  comet  is  a  big  one,  a 
little  tail  begins  to  form.  After  the  comet  whirls  around  the 
sun  it  again  grows  smaller  and  fainter,  the  nucleus  gets  dim, 
the  tail  shortens,  and  when  it  is  lost  sight  of  in  the  telescope 
it  looks  much  as  it  did  when  it  was  first  seen. 


FIG.  91.  —  Showing  two  cometary  orbits,  the  one  an  elongated  ellipse, 
the  other  extending  out  we  know  not  how  far.  The  arc  ab  shows  the 
portion  of  each  orbit  within  which  it  is  possible  for  us  to  see  the 
comet.  These  arcs  are  so  near  alike  that  it  is  often  impossible  to  dis- 
tinguish between  them. 

3.  Orbits  of  Comets.  —  You  may  ask  how  it  is  that  astrono- 
mers know  anything  about  the  movement  of  a  comet  after 
it  disappears  from  their  telescope.  The  answer  is  that  they 


COMETS  AND  METEORS  179 

Calculate  its  orbit  by  the  theory  of  gravitation.  Its  posi- 
tion in  the  heavens  is  observed  with  the  greatest  exactness 
from  day  to  day,  and  thus  it  may  be  known  how  fast  it  is 
moving  at  any  particular  point  of  its  orbit  where  it  is  visible. 
In  this  way  it  may  be  calculated  how  far  it  will  go  before  the 
attraction  of  the  sun  can  stop  it  and  bring  it  back  again. 
There  is  at  each  distance  from  the  sun  a  certain  velocity  which 
the  comet  might  acquire  if  it  fell  from  an  infinite  distance.  If 
the  comet's  velocity  exceeds  this  the  sun  could  never  stop  it,  but 
it  would  move  away  into  infinite  space,  forever.  This  velocity 
is  less  the  farther  the  comet  is  from  the  sun.  At  the  distance 
of  the  earth's  orbit  the  limiting  velocity  is  about  twenty-six 
miles  a  second.  If  it  should  pass  the  earth's  orbit  with  a 
higher  speed  than  this,  we  should  know  that  it  would  never 
return. 

Commonly  the  velocity  is  so  near  this  limit  that  it  is  impos- 
sible, from  the  velocity  alone,  to  be  quite  sure  whether  it  will 
return  or  not.  But  as  there  is  no  evidence  of  a  comet  ever 
exceeding  this  velocity,  it  is  believed  that  every  comet  belongs 
to  the  solar  system  and  will  return  some  time.  In  the  large 
majority  of  cases,  however,  the  return  will  not  take  place  for 
several  thousand  years. 

How  a  Comet  may  have  its  Orbit  Changed.  —  Very  often  a 
comet,  in  falling  toward  the  sun,  or  leaving  it  again,  passes  so 
near  some  great  planet,  generally  Jupiter,  that  the  attraction 
of  the  latter  completely  changes  its  orbit,  by  retarding  the 
comet  so  that  the  sun  can  hold  it  in  a  smaller  orbit.  Then  the 
comet  will  describe  an  ellipse  around  the  sun  with  a  period  of 
a  few  years.  It  is  probable  that  several  of  the  comets  of  short 
period  have  had  their  orbits  changed  in  this  way  by  the  action 
of  Jupiter. 

Twenty  or  thirty  periodic  comets  are  known,  and  new  ones 
are  made  periodic  from  time  to  time,  in  the  way  just  described. 
The  last  case  of  the  kind  was  that  of  a  comet  which  appeared 
in  1889,  known  as  Brooks' s  comet,  after  the  astronomer  by 
whom  it  was  first  seen..  This  body  is  still  revolving  in  a 


180  ASTRONOMY 

period  of  about  seven  years.     It  returned  in  1896  And  will 
return  again  about  1903,  1910  and  so  on. 

When  a  comet  has  thus  been  made  periodic  by  the  attraction 
of  Jupiter,  it  is  liable  to  meet  that  planet  again  at  some  future 
time  and  to  have  its  orbit  again  changed.  Its  period  may  be 
shortened  again,  or  lengthened,  or  the  planet  may  give  it  a 
swing  that  will  send  it  off  from  the  sun  altogether,  so  that 
we  shall  never  again  see  it.  This  happened  to  a  comet  dis- 
covered by  Lexell  in  1770. 

4.  Remarkable  Comets.  —  In  former  times  comets  excited 
great  fear  because  they  were  supposed  to  portend  pestilence, 
war,  the  death  of  kings,  or  other  calamitous  events.  Naturally 
the  more  striking  and  brilliant  the  comet  the  greater  the 
terror  thus  excited.  But  when  the  astronomers  found  that 
these  bodies  moved  according  to  the  law  of  gravitation,  and  had 
no  power  of  their  own,  these  fears  vanished. 

Halley's  Comet.  — This  comet  is  one  of  the  most  remarkable 
in  history ;  not  because  it  was  very  bright,  but  because  it  was 
the  first  one  found  to  be  periodic.  It  was  seen  in  1682. 
Halley,  an  eminent  English  mathematician,  computed  the 
position  of  the  orbit  and  found  that  it  moved  in  nearly  the 
same  orbit  as  a  comet  observed  by  Kepler  in  1607.  He  there- 
fore suggested  that  it  might  come  back  about  every  75  or  76 
years.  Subtracting  76  years  from  1607  brings  us  back  to 
1531.  In  this  year  a  comet  had  actually  been  seen.  Again 
subtracting  75  years  carries  us  back  to  1456.  It  is  known 
from  history  that  in  that  year  a  comet  appeared  which  caused 
such  terror  that  the  Pope  ordered  prayers  to  be  offered  for 
protection  against  the  Turks  and  the  comet.  It  is  supposed 
that  this  gave  rise  to  the  popular  myth  of  the  Pope's  bull 
against  the  comet. 

Halley  now  ventured  the  prediction  that  this  object  would 
return  again  about  1758.  During  this  time  the  mathematical 
methods  of  calculating  the  motion  of  such  a  body  were  in- 
vented. Clairaut,  one  of  the  first  mathematicians  of  France, 


COMETS  AND  METEORS 


181 


calculated  the  effect  which  would  be  produced  by  the  attrac- 
tions of  Jupiter  and  Saturn,  and  found  that  the  comet  would 
be  so  delayed  that  it  would  not  reach  its  perihelion  until 
about  April,  1759.  It  actually  did  pass  its  perihelion  on 
March  1,  1759,  so  that  Clairaut  was  within  a  month  of  the 
truth. 


FIG.  92.  —  Orbit  of  Halley's  comet  crossing  orbits  of  planets. 


Seventy-six  years  more  were  to  elapse,  when  the  comet 
would  again  be  expected.  During  this  time  great  improve- 
ments were  made  in  the  methods  of  computing  the  attraction 
of  the  planets  on  such  a  body.  So  successful  were  the  astrono- 
mers in  this  work  that,  long  before  the  comet  was  seen,  they 
predicted  that  it  would  pass  its  perihelion  early  in  November, 
1835.  It  was  seen  three  or  four  months  before  this  time,  and 
actually  did  pass  the  perihelion  only  three  or  four  days  after 
the  time  of  the  best  prediction.  The  comet  was  followed 
until  May  17,  1836,  when  it  disappeared  from  the  sight  of  the 
most  powerful  telescopes  of  the  time  and  has  not  been  seen 
since.  But  the  astronomer  can  follow  it  with  the  eye  of 
science  almost  as  certainly  as  if  he  saw  it  in  his  telescope. 


182  ASTRONOMY 

It  passed  aphelion,  outside  the  orbit  of  Neptune,  about  1873, 
and  has  been  on  its  way  back  ever  since.  An  exact  calculation 
of  its  return  has  not  yet  been  made,  but  it  is  expected  about 
the  year  1911. 

The  Great  Comet  of  1843. — This  comet  burst  suddenly  into 
view  in  the  neighborhood  of  the  sun  toward  the  end  of  Feb- 
ruary, 1843.  What  was  most  remarkable  was  that  it  was 
visible  in  full  daylight,  so  that  some  observers  actually 
measured  the  angle  between  it  and  the  sun.  It  was  watched 
until  the  middle  of  April,  when  it  disappeared.  It  passed 
nearer  the  sun  than  any  other  body  was  ever  known  to  pass,  — 
so  near  that  with  a  very  slight  change  of  its  original  motion,  it 
would  actually  have  struck  the  sun.  So  far  as  observations 
show,  it  will  not  return  again  for  several  hundred  and  perhaps 
several  thousand  years. 


FIG.  93.  — Telescopic  view  of  the  head  of  Donati's  comet. 

Donati's  Comet  of  1858.  —  This  comet  was  one  of  the  most 
magnificent  on  record.  It  was  discovered  by  Donati,  of 
Florence,  on  June  2,  1858,  and,  when  first  seen,  seemed  to  be 
merely  a  cloudy  patch  of  light.  No  tail  was  noticed  until  the 


COMETS  AND  METEORS  183 

middle  of  August,  and,  at  the  end  of  that  month,  the  comet 
itself  was  barely  visible  to  the  naked  eye.  As  it  approached 
the  sun,  in  September,  it  brightened  up  with  great  rapidity. 
During  the  first  half  of  October,  its  tail  was  40°  in  length,  and 
of  a  curious  featherlike  form. 

The  period  of  this  comet  is  found  to  be  1850  years.     It  will 
therefore  return  again  about  the  thirty-ninth  century. 


FIG.  94.  —  Naked-eye  view  of  Donati's  comet. 

The  Great  Comet  of  1882.  — The  last  very  bright  comet  which 
appeared  was  that  of  the  year  1882.  It  was  remarkable,  not 
only  for  its  size  and  splendor,  but  from  the  fact  that  it  moved 
very  nearly  in  the  orbit  of  the  comet  of  1843,  swinging 
dangerously  near  the  sun,  as  that  comet  did. 

Among  the  comets  of  short  period,  and  those  whose  history 
is  most  interesting,  are  those  of  Encke  and  Biela. 

Encke's  Comet.  —  This  cornet  was  first  recognized  as  periodic 
in  1818,  when  it  was  found  by  the  astronomer  Pons  of  Mar- 
seilles, France.  On  computing  its  orbit  he  found  that  it  had 
been  seen  three  times  before,  the  first  time  in  1786.  But  the 
former  observers  did  not  know  that  it  was  the  same  comet 


184 


ASTRONOMY 


which,  had  reappeared.     It  was  now  found  to  be  revolving 
round  the  sun  in  a  period  of  about  1200  days,  or  between  3 


FIG.  95.  —  A  telescopic  comet.     Three  views  of  Encke's  comet  as  drawn 
by  Vogel  in  1871. 

and  4  years.     It  has  been  followed  from  time  to  time,  dur- 
ing most  of  its  returns  to  perihelion,  ever  since.     The  most 


COMETS  AND  METEORS  185 

thorough  investigation  of  its  orbit  was  made  by  the  German 
astronomer  Encke,  whose  name,  for  this  reason,  is  given  to 
the  comet. 

The  most  remarkable  discovery  of  Encke  was  that  the  comet 
seems  to  be  gradually  shortening  its  period  of  revolution.  At 
most  of  its  returns  it  seems  to  be  accelerated  a  few  hours. 
This  is  supposed  to  arise  from  the  comet  passing  through  some 
extremely  rare  medium  which  resists  its  motion  when  it  passes 
nearest  to  the  sun.  In  consequence  its  orbit  is  growing  smaller, 
and  thus  the  revolution  is  completed  in  a  shorter  time.  As  no 
other  comet  shows  similar  acceleration,  it  is  not  regarded  as 
quite  certain  that  a  resisting  medium  is  the  real  cause  of  the 
change. 

Biela's  Comet.  —  The  early  history  of  this  comet  is  a  good 
deal  like  that  of  Encke's.  It  was  seen  in  1826  by  an  Austrian 
named  Biela.  On  computing  its  orbit  it  was  found  to  be 
periodic.  On  calculating  its  motions  back  it  was  found  to  have 
been  observed  in  1772,  and  again  in  1805.  But  both  the  pre- 
vious observers  thought  the  comet  to  be  new. 

Its  time  of  revolution  was  now  fixed  at  6  years  and  8  months. 
In  1846  it  again  returned  to  perihelion,  and  was  found  to  have 
suffered  an  accident  never  before  known  in  the  case  of  a 
heavenly  body.  It  had  separated  into  two  parts,  so  that, 
instead  of  one  comet,  two  were  seen. 

At  its  next  return,  in  1852,  both  parts  were  observed.  But 
the  comet  has  never  been  seen  since,  though  repeatedly  looked 
for  at  the  proper  times.  Its  parts  have  all  been  dispersed,  but 
are  doubtless  revolving  round  the  sun  as  minute  invisible 
particles,  as  we  shall  presently  explain. 

5.  Constitution  of  Comets.  —  From  what  we  have  said,  it  is 
plain  enough  that  the  coma  and  tail  of  a  comet  are  neither 
solid  nor  liquid,  but  are  composed  of  an  exceedingly  thin 
vapor,  lighter  and  thinner  than  the  finest  vapor  on  the  surface 
of  the  earth.  This  vapor  seems  to  rise  from  the  nucleus  of  the 
comet  under  the  influence  of  the  sun's  rays,  much  as  heat  makes 


186  ASTRONOMY 

a  pot  of  water  boil.  But  the  vapor  is  enormously  thinner  than 
any  steam  rising  from  a  pot.  It  forms  a  cloud  around  the 
nucleus,  and  moves  along  with  it.  As  the  comet  approaches 
the  sun,  the  vapor  is,  in  some  way  not  yet  fully  understood, 
repelled  by  the  sun,  and  thus  driven  off  in  a  stream.  This 
stream  is  the  tail  of  the  comet,  and  is  always  directed  away 
from  the  sun,  thus  showing  that  the  sun  repels  instead  of 
attracting  the  matter  that  composes  it.  Hence  the  tail  is  not 
an  appendage  which  the  comet  carries  with  it,  like  a  bird 
carries  its  tail,  but  is  rather  like  a  stream  of  smoke  rising 
from  a  chimney. 

It  follows  from  this  that  a  comet  which  shows  a  real  tail  is, 
when  near  the  sun,  continually  losing  its  substance  by  evapora- 
tion. When  we  consider  how  very  large  the  tail  is,  nearly 
always  many  millions  of  miles  in  length,  we  might  suppose 
that  the  matter  of  the  comet  must  be  evaporating  very  fast. 
But  the  actual  amount  of  matter  in  the  tail  of  a  comet  is 
exceedingly  small.  The  tail  is  so  tenuous  that  the  stars  can 
be  seen  through  millions  of  miles  of  it  without  any  diminu- 
tion of  their  light.  It  reflects  so  little  of  the  sun's  light 
that  it  cannot  be  seen  at  all  in  the  daytime  or  in  bright  twi- 
light It  is  quite  possible  that  there  may  be  only  a  few 
pounds  of  matter  in  the  whole  tail  of  a  comet. 

Still,  all  the  matter  that  can  evaporate  from  the  comet  will 
in  time  be  lost.  Thus  it  happens  that  the  tails  of  the  periodic 
comets  gradually  diminish  as  they  make  their  successive  revo- 
lutions. This  is  because  the  volatile  matter  is  gradually  being 
evaporated.  Those  comets  which  have  made  a  great  many 
revolutions  scarcely  show  any  tail  at  all. 

One  result  of  this  is  that  periodic  comets,  like  men,  have 
only  a  limited  time  in  which  to  exist.  They  grow  thinner  and 
paler  as  they  make  theif  revolutions,  and  are  probably  nearly 
all  doomed  at  some  time  to  disappear.  Indeed,  several  have 
disappeared  during  the  last  50  years.  But  new  ones  are  being 
brought  in  by  the  attraction  of  Jupiter  to  take  their  places  in  the 
way  already  described,  so  that  the  number  is  as  great  as  ever. 


COMETS  AND  METEORS  187 

6.  Meteors.  —  Every  one  who  looks  at  the  heavens  with  care 
by  night  must  notice  the  meteors,  or  shooting  stars,  of  which 
several  can  be  seen  on  any  clear  night  of  the  year.  These 
objects  suddenly  come  into  view,  like  a  moving  star,  dart 
swiftly  along  for  a  moment,  and  then  disappear.  Their  cause 
has  only  recently  been  discovered. 

It  is  now  known  that,  besides  the  bodies  of  the  solar  system 
which  we  see  with  our  telescopes,  there  are  countless  millions 
of  little  particles  of  matter  revolving  around  the  sun  in  orbits 
of  various  forms  and  sizes.  These  particles  are  so  small  that 
no  telescope  is  powerful  enough  to  show  them.  As  the  earth 
flies  along  in  its  orbit  it  is  constantly  encountering  these  little 
particles,  which,  of  course,  first  strike  the  atmosphere  by  which 
the  earth  is  surrounded.  Now  it  is  a  law  of  physics  that,  when 
a  body  moves  with  exceeding  swiftness  through  the  air,  heat  is 
generated  by  the  friction  and  resistance.  The  great  velocity 
of  these  particles  moving  around  the  sun  —  a  velocity  of  many 
miles  a  second  —  results  in  so  much  heat  and  friction  by  the  air 
that  the  particle  is  almost  instantly  destroyed  or  burnt  with  a 
great  evolution  of  light  and  heat.  Then  those  who  see  this 
light  call  it  a  meteor  or  shooting  star.  The  invisible  particles, 
as  they  exist  before  striking  the  air,  are  called  meteoroids. 

The  meteoroids  are  very  different  in  size.  Hence  shooting 
stars  are  of  very  different  degrees  of  brilliancy.  Sometimes 
the  meteoroids  are  so  large  that  they  rush  a  long  distance 
through  the  air  before  they  are  burnt  up  by  the  fervent  heat. 
Then  we  see  a  very  brilliant  meteor,  and  sometimes  hear  a 
loud  report  like  thunder,  or  the  discharge  of  artillery.  This 
is  caused  by  the  concussion  of  the  air  by  the  meteoroid. 

Occasionally  the  latter  is  so  large  that  it  falls  to  the  ground 
before  being  burnt  up.  It  is  then  called  a  meteorite.  If 
examined  immediately  after  it  falls,  it  is  found  to  be  very  hot 
on  the  surface,  perhaps  partly  melted,  in  consequence  of  its 
rapid  passage  through  the  air.  Many  meteorites  have  fallen 
in  this  way  and  some  of  them  are  preserved  in  our  museums. 
They  are  found  to  consist  very  largely  of  iron  and  stone. 


188  ASTRONOMY 

It  is  not  certain  whether  the  meteoroids  which  form  the 
ordinary  shooting  stars  are  composed  of  the  same  substance 
as  these  large  masses  that  fall  to  the  ground.  When  they  dis- 
appear in  the  high  regions  of  the  air,  they  are  completely 
dissipated,  so  that  it  is  impossible  to  find  any  remains  of  them. 

7.  Meteoric  Showers.  —  Sometimes  shooting  stars  appear 
much  greater  in  numbers  than  usual.  They  may  follow  each 
other  so  rapidly  that  they  can  scarcely  be  counted.  Then 
there  is  said  to  be  a  meteoric  shower.  Several  remarkable 
showers  of  this  kind  are  recorded  in  history.  One  occurred 
in  the  year  1799  and  another  in  the  year  1833.  A  third,  less 
striking,  was  seen  in  1866.  All  occurred  at  the  same  time  of 
the  year,  about  November  14. 

It  is  now  known  that  these  showers  are  caused  by  an  immense 
stream  of  meteoroids  which  revolve  around  the  sun  in  a  very 
elongated  orbit,  making  about  three  revolutions  in  a  century. 
The  swarm  is  so  long  that  although  millions  are  passing  all 
the  time,  it  takes  two  or  three  years  for  the  whole  procession 
to  pass  a  given  point.  It  happens  that  the  orbit  of  this  swarm 
intersects  the  orbit  of  the  earth  at  that  point  where  the  earth 
passes  about  the  middle  of  November  of  every  year.  Thus, 
during  the  two  or  three  years  that  the  swarm  is  passing,  the 
earth  will  encounter  it  two  or  three  times  in  succession. 
Then,  when  the  swarm  has  got  by,  nothing  more  will  be  seen 
of  these  meteors,  except  perhaps  a  few  stragglers. 

It  is  known  that  a  comet  is  revolving  in  the  same  orbit  with 
these  meteoroids  that  cause  the  November  showers.  Hence  it 
is  supposed  that  the  meteoroids  belong  to  the  comet  and  were 
once  a  part  of  it. 

Radiant  Point  of  a  Meteoric  Shower.  —  Each  meteor  in  a  shower 
of  course  describes  a  certain  path  on  the  celestial  sphere.  If 
we  continue  each  of  these  paths  in  the  reverse  direction  to 
that  of  the  motion  of  the  meteor,  we  shall  find  them  all 
directed  from  one  and  the  same  point  in  the  heavens.  This  is 
called  the  radiant  point  of  the  shower. 


COMETS  AND  METEOR& 


189 


190  ASTRONOMY 

The  radiant  point  is  an  effect  of  perspective.  The  real  paths 
described  by  the  meteors  are  parallel  and  nearly  straight  lines, 
which,  however,  look  to  us  like  circles  on  the  celestial  sphere. 
The  radiant  point  is  simply  that  point  of  the  sphere  from 
which  the  lines  are  directed. 

The  August  Showers.  —  Although  the  November  showers 
which  we  have  described  are  the  most  remarkable  recorded, 
it  is  found  that  there  are  other  nights  of  the  year  in  which 
meteors  are  seen  in  unusual  numbers.  One  of  these  dates  is 
about  the  9th  or  10th  of  August.  About  this  time  every  year, 
if  one  sits  up  until  midnight,  he  will  see  an  unusual  number  of 
shooting  stars,  which  mostly  move  from  the  northeast  toward 
the  southwest.  They  are  distinguished  by  leaving  trails  behind 
them  which  last  for  a  few  seconds.  They  are  called  Perseids 
because  their  radiant  point  is  in  the  constellation  Perseus. 


CHAPTER  XIV 
THE  CONSTELLATIONS 

1.  About  the  Stars  in  General.  —  In  the  most  ancient  times, 
the  priests  and  astronomers  who  watched  the  sky  by  day  and 
night  saw  that  the  heavenly  bodies  were  of  two  kinds  in 
respect  to  their  motions.  Seven  of  them,  the  Sun,  the  Moon, 
Mercury,  Venus,  Mars,  Jupiter,  and  Saturn,  seemed  to  have 
separate  motions  of  their  own.  Hence  they  were  called  plan- 
ets, from  the  Greek  word  planetes,  a  wanderer.  This  is  the 
origin  of  the  word  planet.  The  remainder,  of  which  one  or  two 
thousand  were  easily  seen,  all  seemed  fixed  in  the  celestial 
sphere.  This  sphere  was  supposed  to  turn  on  its  axis  every 
day,  carrying  these  bodies  with  it.  This  is  why  the  latter 
were  called  fixed  stars.  The  fixed  stars  comprise  all  the  stars 
we  see  in  the  heavens  at  night,  the  planets  excepted. 

We  have  already  shown  that  the  seven  planets  of  the  ancients 
belong  to  the  solar  system,  while  the  stars  are  many  thousand 
times  farther  from  us  than  the  planets  are.  Let  us  try  to  gain 
an  idea  of  these  distances.  There  are  in  the  heavens  three  or 
four  pairs  of  stars  so  close  together  that  only  a  keen  eye  can 
see  that  there  are  two  stars.  If  a  railway  train  could  run  at  a 
speed  of  60  miles  an  hour  from  one  of  these  stars  to  the  other, 
without  ever  stopping,  it  might  require  a  million  of  years  for 
the  journey.  During  the  5000  years  since  the  beginning  of 
written  history,  the  journey  would  hardly  have  been  com- 
menced. It  would  be  as  if  a  train  which  was  to  run  to  a  city 
several  miles  away  was  just  pulling  out  of  the  station. 

Each  of  the  stars  which  has  been  carefully  observed  is  mov- 
ing forward  in  what  seems  to  us  a  straight  line,  often  at  the 

181 


192  ASTRONOMY 

rate  of  many  miles  per  second,  and  the  same  is  probably  true 
of  all.  This  motion  of  the  stars  is  called  proper  motion.  Owing 
to  the  great  distance  of  the  stars,  it  cannot  be  seen  except  by 
telescopic  observation.  It  will  be  described  subsequently. 

The  stars  are  bodies  like  our  sun,  surrounded  by  layers  of 
heated  vapor,  and  shining  by  their  own  light. 

They  are  very  different  in  real  brightness.  Some  are  hun- 
dreds or  thousands  of  times  brighter  than  our  sun ;  as  I  have 
already  said,  they  look  small  because  they  are  so  far  away. 

They  are  at  very  different  distances.  The  most  distant  that 
we  know  are  hundreds  or  even  thousands  of  times  farther  than 
the  nearest. 

Number  of  Stars.  —  In  the  whole  heaven  there  are  about  5000 
stars  that  can  be  seen  by  an  ordinarily  good  eye.  But  as  one 
half  of  these  are,  at  any  one  moment,  below  the  horizon,  and 
many  of  the  other  half  so  near  the  horizon  that  the  fainter  ones 
cannot  be  seen,  it  is  not  to  be  expected  that  more  than  1500  to 
2000  can  ever  be  seen  at  the  same  time. 

For  every  star  visible  to  the  naked  eye,  there  are  thousands 
that  can  be  seen  with  a  telescope,  though  invisible  to  the  naked 
eye.  These  are  called  telescopic  stars.  With  every  improve- 
ment in  the  telescope  more  are  seen,  so  that  the  total  number 
made  known  by  the  largest  instruments  probably  amounts  to 
50  millions.  The  number  that  may  be  photographed  is  yet 
greater,  perhaps  even  100  millions. 

The  Milky  Way.  —  Every  one  who  carefully  looks  at  the  sky 
on  a  clear  night  must  notice  the  Galaxy,  or  Milky  Way,  as  it  is 
commonly  called;  that  beautiful  white  arch  which  spans  the 
heavens.  To  the  naked  eye  it  appears  as  an  irregular  row  of 
cloudlike  masses,  having  a  milky  aspect,  from  which  its  famil- 
iar name  is  given  it.  When  examined  with  a  telescope,  the 
light  is  found  to  come  from  innumerable  stars,  too  faint  to  be 
seen  separately  with  the  naked  eye.  With  a  good-sized  tele- 
scope of  a  low  magnifying  power,  the  field  of  view  is  seen  to  be 
studded  with  such  stars  shining  like  little  diamonds.  The 
total  number  of  telescopic  stars  which  form  the  Milky  Way  is 


THE  CONSTELLATIONS 


193 


FIG.  96.  —  The  Milky  Way  near  the  star  15  Monocerotis.    AE  =  6  h.  35  m. 
Decl.  =  N.  10°.     (Photographed  by  Barnard,  1894  ;  exposure,  3£  hours.) 


NEWCOMR's 


.  —  13 


194  ASTRONOMY 

very  great,  probably  greater  than  the  number  of  all  the  stars 
in  the  remainder  of  the  heavens. 

The  question  how  the  stars  are  arranged  in  the  Milky  Way 
is  one  of  the  most  interesting  with  which  astronomers  concern 
themselves.  With  the  naked  eye  we  can  see  that  they  are  not 
scattered  uniformly,  because  if  they  were  the  Milky  Way 
would  be  everywhere  equally  bright.  Many  of  them  are  col- 
lected into  masses  or  clusters.  Some  of  the  clusters  contain  so 
many  stars  that  it  is  almost  impossible  to  count  them,  because 
they  cannot  be  separated  with  the  most  powerful  telescope ;  in 
fact,  it  is  hardly  possible  to  sweep  a  great  telescope  along  the 
course  of  the  Milky  Way  without  finding  collections  of  beauti- 
ful objects  too  numerous  to  be  separately  described. 

Magnitudes  of  the  Stars.  —  The  stars  are  very  different  in 
apparent  brightness.  This  arises  both  from  their  different 
real  brightness  and  their  different  distances. 

The  ancient  astronomers  divided  all  the  stars  they  were 
able  to  see  into  six  classes,  according  to  their  apparent  bright- 
ness or  magnitude.  At  one  extreme  were  the  fifteen  brightest 
stars.  These  were  called  of  the  first  magnitude.  At  the 
other  extreme  were  the  great  number  of  stars  which  could 
barely  be  seen  with  a  good  eye.  These  were  called  of  the 
sixth  magnitude.  Between  these  extremes  came,  in  regular 
order  of  brightness,  stars  of  the  second,  third,  fourth,  and 
fifth  magnitudes.  The  brightest  six  stars  of  Ursa  Major 
and  the  brightest  four  of  Cassiopeia  are  of  the  second  mag- 
nitude. The  stars  a  degree  fainter  than  these  are  of  the 
third  magnitude;  those  yet  a  degree  fainter  of  the  fourth. 
The  fifth  are  the  faintest  that  one  will  easily  see,  unless 
the  sky  is  clear  and  moonless,  and  his  eyes  are  good. 

The  same  system  of  magnitudes  is  continued  to  the  tele- 
scopic stars.  Thus  we  have  stars  of  the  seventh,  eighth,  ninth, 
and  so  on  up  to  the  fifteenth  or  sixteenth  magnitudes.  The 
latter  are  about  the  faintest  that  can  be  seen  or  photographed 
with  the  most  powerful  telescope.  But  we  do  not  know  how 
many  still  fainter  ones  may  really  exist.  .  With  every  increase 


THE  CONSTELLATIONS  195 

in  the   power  of   the  telescope   new  stars  are  brought  into 
view. 

Colors  of  the  Stars.  —  If  we  carefully  compare  the  stars 
with  each  other,  we  shall  see  that  they  are  perceptibly  differ- 
ent in  color,  ranging  from  the  reddish  tint  of  Aldebaran  to 
the  bluish  white  of  Vega.  This  difference  shows  that  the 
substances  of  which  these  bodies  are  composed,  and  also  their 
temperatures,  are  different.  No  doubt  the  atmospheres  by 
which  they  are  surrounded  absorb  a  part' of  the  light  from 
the  star,  and  thus  change  the  apparent  color.  The  red  stars 
are  supposed  to  be  less  hot  than  the  blue  ones,  and  to  be  sur- 
rounded by  a  dense  atmosphere,  which  absorbs  the  blue  light, 
but  lets  the  red  light  pass. 

2,  How  the  Constellations  and  Stars  are  Named.  —  A  constel- 
lation is  a  large  group  of  stars.  A  duster  is  a  small  group  of 
many  stars  very  close  to  each  other. 

The  stars  are  commonly  divided  among  85  constellations. 
The  majority  of  these  were  imagined  by  the  ancient  astron- 
omers. The  remainder  have  been  mapped  out  in  modern 
times. 

To  most  of  the  older  constellations  are  given  the  names  of 
heroes,  goddesses,  or  animals.  The  outlines  of  these  persons 
or  objects  were  supposed  to  be  drawn  on  the  sky  in  such  a 
manner  that  the  figure  should  include  the  principal  stars  of 
the  constellation.  Thus  we  have  Cassiopeia,  the  Lady  in  her 
Chair ;  Andromeda,  the  Chained  Goddess ;  Auriga,  the  Chari- 
oteer ;  Ursa  Major,  the  Great  Bear ;  etc. 

Special  names,  mostly  given  by  the  Arabian  astronomers, 
are  often  used  to  designate  the  brighter  stars.  But  it  is  now 
common  to  name  a  star  as  we  do  a  person,  by  a  Christian 
name  and  a  surname.  The  Christian  names  for  the  principal 
stars  are  the  letters  of  the  Greek  alphabet,  Alpha  (a),  Beta 
(/?),  Gamma  (y),  etc.  The  surnames  are  the  names  of  the 
constellations  to  which  the  star  belongs.  They  are  generally 
expressed  in  Latin.  This  method  of  naming  the  principal 


196 


ASTRONOMY 


stars  is  called  the  Bayer  system,  after  the  astronomer  Bayer 
who  introduced  it  about  1600. 

Generally,  but  not  always,  the  brightest  star  in  a  constella- 
tion has  the  name  Alpha,  the  next  brightest  the  name  Beta, 
and  so  on. 

Only  the  principal  stars  can  be  named  in  this  way.  To 
others  numbers  are  assigned,  according  to  various  systems. 
The  following  are  the  names  of  some  of  the  principal  stars 
on  the  two  systems.  First  is  given  the  old  name,  generally 
from  the  Arabic,  and  then  the  name  on  the  Bayer  system, 
which  is  now  generally  used  by  astronomers. 


OLD  NAME 

Algenib 

Polaris  1 

The  Pole  Star  / 

Archernar 

Algol 

Alcyone 

Aldebaran 

Canopus 

Sirius 

Castor 

Procyon 

Pollux 

Regulus 

Mizar 

Spica 

Arcturus 

Antares 

Vega 

Altair 

Fomalhaut 

Markab 


BAYER  NAME 
y  Pegasi 
a  Ursae  Minoris 

a  Eridani 

/5  Persel 

17  Tauri 

a  Tauri 

a  Argus 

a  Canis  Majoris 

a  Geminorum 

a  Canis  Minoris 

/3  Geminorum 

a  Leonis 

C  Ursse  Majoris 

a  Virginis 

a  Bootis 

a  Scorpii 

a  Lyrse 

a  Aquilse 

a  Piscis  Australis 

a  Pegasi 


When  translated  these  Bayer  names  would  read  Gamma  of 
Pegasus,  Alpha  of  the  Little  Bear,  and  so  on,  Pegasus,  the 
Flying  Horse,  Ursa  Minor,  the  Little  Bear,  etc.,  being  the 
names  of  the  constellations. 


THE  CONSTELLATIONS 


197 


3.  Description  of  the  Principal  Constellations.  —  We  shall  now 
describe  the  most  noteworthy  constellations,  clusters,  and 
other  remarkable  objects  in  the  heavens  which  can  be  seen 

without  a  telescope.  He  who  enters 
on  this  study  should  look  at  the 
heavens  for  himself  on  as  many  clear 
and  moonless  nights  as  possible.  It 
is  well  to  begin  with  the  constella- 
tions around  the  north  pole,  because 
in  our  latitudes  they  can  be  seen  on 
almost  every  clear  night  of  the  year. 
These  are  called  Circumpolar  constel- 
lations. 

We  have  already  described  the  two 
principal  circumpolar  constellations, 
Ursa  Major,  or  the  Great  Bear,  and 
Cassiopeia.     Between  these  two  and 
in  the  immediate  neighborhood  of  the 
pole  lies  Ursa  Minor, 
It  contains 
Alpha 


FIG.  97.  —  The  Great  Bear 
or  Dipper. 


•  Pole  Star 


to  which  the  pole  star  belongs, 
two   stars   of    the   second  magnitude. 
Ursce  Minoris,   or  the  pole   star,  is  near  the 
end  of  the  animal's  tail ;  Beta  is  in  his  body. 
Draco,  the  dragon,  is  represented   as  a  long, 
snakelike    figure,    whose    body   curves    round 
between  Ursa  Major  and  Ursa  Minor. 

The  visibility  of  other  constellations  will 
depend  upon  the  time  of  year  and  time  of  day 
at  which  we  look.  We  may  divide  our  views  FIG.  98.  — OJrsa 
into  spring,  summer,  autumn,  and  winter  views. 
This  does  not  mean  that  we  can  get  the  views 
only  at  these  respective  seasons,  but  that  at  these  seasons  the 
stars  will  be  in  a  certain  position  early  in  the  evening,  when  it 
is  convenient  to  look  at  them.  You  can  see  the  stars  which 
we  call  the  autumn  ones  in  the  spring,  if  you  will  get  up 
before  daylight  in  the  morning. 


Minor  or  Little 
Bear. 


198 


ASTRONOMY 


4.   Constellations  visible  in  the  Evenings  of  February  and  March. 

—  In  the  spring  we  shall  see  the  Milky  Way  spanning  the 
heavens,  and  passing  west  of  the  zenith  and  over  to  the  south. 
We  shall  first  notice  the  bright  constellations  which  lie  in  its 

course.  In  the  northwest  we  shall 
see  Cassiopeia.  Above  it  is  Perseus, 
which  can  be  recognized  by  a  row 
of  stars  running  along  the  center 
of  the  Milky  Way,  one  of  which,  of 
the  second  magnitude,  is  much 
brighter  than  the  rest.  This  is 
called  Alpha  Persei. 

In  this  constellation  you  will  see 
a  remarkable  cluster  which  looks, 
to  the  naked  eye,  like  a  cloudy  mass.  This  is  called  the 
cluster  of  Perseus.  It  forms  the  hilt  of  the  sword  which 
the  hero  Perseus  wears  when  he  is  painted  on  the  sky.  Even 
with  a  good  spyglass  one  can  see  that  this  cloudy  mass  con- 
sists of  a  collection  of  very  small  stars. 


FIG.  99.  —  Cassiopeia. 


\ 


FIG.  100.  —  Aldebaran  and  the 
Hyades. 


FIG.  101.  —  The  Pleiades,  or 
the  Seven  Stars. 


Above  and  south  of  Perseus  lies  Auriga,  the  Charioteer. 
It  contains  a  star  of  the  first  magnitude  called  Capella,  the 
Goat,  which  will  now  be  west  of  the  zenith. 

West  of  the  zenith  and  below  the  Milky  Way  is  the  constel- 
lation Taurus,  the  Bull,  which  may  be  recognized  by  the  bright 


THE  CONSTELLATIONS  199 

red  star  Aldebaran,  which  is  near  one  eye  of  the  Bull,  and  used 
to  be  called  the  Bull's  Eye.     It  is  at  one  end  of  two  lines  of 


FIG.  102.  —  Telescopic  view  of  the  Pleiades,  after  Engelmann.  The  six 
brightest  stars  are  easily  seen  by  the  naked  eye  ;  the  four  or  five  next 
in  size  are  also  visible  to  very  good  eyes. 

stars,  forming  a  figure  like  the  letter  V,  called  the  Hyades. 
One  arm  of  the  V  is  a  very  pretty  close  row  of  stars. 

In  the  same  constellation,  a  little  to  the  northwest,  you  see 
the  Pleiades,  or  "  seven  stars/'  a  cluster  which  is  familiar  to 


200 


ASTRONOMY 


all.  In  reality  only  six  stars  of  this  cluster  can  be  plainly 
seen  with  the  naked  eye.  A  good  eye  on  a  clear  moonless 
night  may  see  five  more,  making  eleven.  There  is  an  old  tra- 
dition that  the  cluster  originally  had  seven  stars,  of  which  one 
disappeared.  .  This,  however,  is  probably  a  myth.  With  a 
telescope  many  additional  stars  may  be  seen  in  this  cluster. 

East  from  Taurus,  on  the  other  side  of  the  Milky  Way  and 
near  the  zenith,  lie  Gemini,  the  Twins.  They  are  recogniz- 
able by  two  stars  of  the 
first  magnitude,  Castor 
and  Pollux,  each  of  which 
marks  a  head  on  one  of 
the  Twins. 

Below  Castor  and  Pol- 
lux we  see  Canis  Minor, 
the  Little  Dog,  marked 
by  a  star  of  the  first 
magnitude,  Procyon. 

Across  the  Milky  Way 
from  Canis  Minor,  and 
west  of  the  meridian,  we 
see  Orion,  the  most  bril- 
liant constellation  in  the 
heavens.  It  contains  two 


i 


FIG.  103. —Orion.  stars  of  tne  nrst  magni- 

tude, called    Alpha    and 

Beta  on  our  modern  system,  but  Betelguese  and  Rigel  on  the 
old  system.  The  former  is  a  reddish  star  marking  the  shoul- 
der of  Orion.  The  latter  is  a  bright  bluish  white  star,  farther 
below,  marking  his  left  foot.  A  row  of  three  bright  stars 
between  these  two  marks  his  belt,  and  three  more  in  a  vertical 
line  below  show  his  sword.  His  head  is  formed  by  a  cluster 
of  three  little  stars  to  the  right  of  Betelguese. 

Southeast  of  Orion,  on  the  western  border  of  the  Milky  Way, 
is  Canis  Major,  the  Great  Dog,  remarkable  for  containing 
Sirius,  the  brightest  fixed  star  in  the  heavens. 


THE  CONSTELLATIONS  201 

On  the  southern  horizon  will  lie  Argo  Navis,  the  Ship  Argo, 
a  very  large  and  celebrated  constellation,  of  which  the  greater 
part  never  rises  in  our  northern  latitudes.  It  contains  the 
bright  star  Canopus,  the  second  brightest  in  the  heavens, 
which  in  our  country  can  be  seen  only  in  the  southern  states. 

Another  constellation  is  Aries,  the  Ram,  which  will  be 
setting  in  the  west.  It  may  be  recognized  by  three  stars  of 
the  second,  third,  and  fourth  magnitude,  forming  an  obtuse 
triangle. 

East  of  Gemini  will  be  seen  Cancer,  the  Crab,  which  contains 
no  bright  stars,  but  is  remarkable  for  Prcesepe,  which  looks  to 
the  naked  eye  like  a  little  faint  spot  of  milk  in  the  sky.  A 
very  small  telescope  shows  it  to  be  a  cluster  of  stars. 

Still  farther  east  is  seen  Leo,  the  Lion,  which  is  marked  by 
the  bright  star  Regulus.  North  of  it  will  be  seen  a  curved  row 
of  stars  forming  a  figure  like  a  sickle,  of  which  Regulus  is  the 
handle. 

5.  The  Early  Summer  Constellations.  —  The  next  view  of  the 
heavens  whicli  we  shall  describe  is  that  in  the  evening  of  May, 
or  an  early  evening  of  June.  The  Milky  Way  passes  so  near 
the  horizon  that  it  will  probably  be  invisible. 

In  the  west  can  be  seen  Castor  and  Pollux  and  Procyon,  but 
the  other  brilliant  constellations  are  near  the  horizon  or  below  it. 

We  see  Leo,  just  described,  a  little  west  of  the  meridian. 

A  little  east  of  the  meridian  will  be  seen  Virgo,  the  Virgin, 
in  the  south,  which  has  a  bright  star  Spica,  about  as  bright  as 
Regulus. 

Scorpius,  the  Scorpion,  will  be  low  down  in  the  southeast, 
having  just  risen.  We  shall  describe  it  later. 

Quite  near  the  zenith  we  shall  see  Coma  Berenices,  the  Hair 
of  Berenice.  This  is  a  very  large  thin  cluster  of  faint  stars, 
quite  irregular  in  form. 

East  of  this  cluster  we  see  Bootes,  the  Herdsman.  It  is 
marked  by  Arcturus,  a  very  brilliant  reddish  star,  mentioned 
in  the  Book  of  Job.  In  the  mythological  arrangement  of  the 


202  ASTRONOMY 

constellations,  Bootes  is  represented  as  holding  two  dogs  in  a 
leash.     They  are  called  Canes  Venatiti,  and  are  chasing  Ursa 

Major  round  the  pole. 

In  the  northeast,  below  Arcturus, 
we  see  Corona  Borealis,  the  North- 
ern Crown,  a  small  but  exceedingly 
beautiful  chaplet  of  stars,  which 
we  can  easily  imagine  to  form  a 
crown.  The  brightest  of  them  is 
of  the  second  magnitude,  and  is 
called  Alpha  Coronse  Borealis. 


*  6.   The  August   Constellations. - 

the  Northern  Crown. 

It  we  look  in  the  latter  part  of  the 

evening,  during  the  month  of  August,  or  in  the  early  even- 
ing during  the  first  part  of  September,  we  shall  again  see 
the  Milky  Way  spanning  the  heavens  like  an  arch.  But  we 
now  see  a  different  part  from  what  we  saw  in  the  early  spring. 
Arcturus  is  now  sinking  in  the  west,  while  Cassiopeia  is  rising 
in  the  northeast.  In  the  Milky  Way,  some  distance  above 
Cassiopeia,  we  see  Cygnus,  the  Swan.  It  has  five  stars  arranged 
in  the  shape  of  a  cross,  which  form  the  wings,  head,  and  tail  of 
the  swan.  The  brightest  of  these,  Alpha  Cygni,  is  nearly  of  the 
first  magnitude. 

South  from  Cygnus  and  near  the  zenith,  is  Lyra,  the  Harp, 
a  small  but  very  beautiful  constellation.  It  contains  Vega,  a 
brilliant,  bluish-white  star  of  the  first  magnitude.  South  of 
Vega  are  four  stars  of  the  fourth  magnitude  forming  an  oblique 
parallelogram,  by  which  the  constellation  can  be  recognized. 

East  of  Vega,  and  very  close  to  it,  is  the  star  Epsilon  Lyrae, 
which,  if  you  have  a  very  keen  eye,  you  will  see  to  be  composed 
of  two  stars  very  close  together. 

South  of  Lyra  and  in  the  Milky  Way,  we  next  look  for 
Aquila,  the  Eagle.  It  is  readily  found  by  the  bright  star  Altair, 
or  Alpha  Aquila,  of  the  first  magnitude,  situated  between  two 
smaller  stars. 


THE  CONSTELLATIONS 


203 


Northeast  of  Aquila  is  Delphinus,  the  Dolphin,  familiarly 
known  as  Job's  Coffin.  Its  four  principal  stars  are  arranged 
in  the  form  of  a  lozenge. 


FIG.  105.  — Lyra,  the  Harp. 


FIG.  106.  —Aquila,  the  Eagle. 


Scorpius,  the  Scorpion,  is  now  low  down  in  the  south,  west 
of  the  meridian.  Its  brightest  star  is  Antares,  or  Alpha 
Scorpii,  which  is  red  in  color  and  nearly  of  the  first  magni- 


FIG.  107. — Scorpius,  the  Scorpion, 
with  Antares,  the  brightest  star. 


FIG.  108.  —  Delphinus,  the 
Dolphin,  or  Job's  Coffin. 


tude.     West  of  it  is  a  long  curved  row  of  stars  forming  the 
head  and  claws  of  the  scorpion. 

East  of  Scorpius  we  see  Sagittarius,  the  Archer. 


204  ASTRONOMY 

7.  The  November  Constellations.  —  We  now  see  the  Milky 
Way  a  little  north  of  the  zenith,  seeming  to  rest 'on  the  east 
and  west  horizon.  All  the  constellations  that  we  see  in  its 
course  have  already  been  described. 

No  bright  stars  are  visible  except  some  of  those  already 
mentioned.  Lyra  is  in  the  northwest;  Aquila,  in  the  west. 
South  of  the  zenith  we  see  the  Square  of  Pegasus,  four  stars 
of  the  second  magnitude  forming  the  corners  of  a  large  square. 
Three  of  them  belong  to  the  constellation  Pegasus,  the  Flying 
Horse. 

In  the  south,  or  southwest,  near  the  horizon,  we  see  the  star 
FomalJiaut,  belonging  to  the  constellation  Piscis  Australis,  the 
Southern  Fish. 

The  zodiacal  constellations,  Capricornus,  the  Goat,  Aquarius, 
the  Water  Bearer,  and  Pisces,  the  Fishes,  are  all  visible  in  the 
south  or  southwest,  but  they  do  not  contain  any  very  bright 
stars.  Aries,  the  Ram,  is  high  up,  southeast  of  the  zenith,  and 
Taurus,  with  the  Pleiades,  is  seen  lower  down  in  the  east; 
Castor,  Pollux,  and  Procyon,  lower  yet. 

If  we  wait  till  ten  o'clock,  we  shall  see  Orion  rise  to  the  south 
of  east,  and  after  him  Sirius. 


CHAPTER  XV 
THE  STARS  AND  NEBULA 

1.  The  Stars  are  Suns.  —  The  difference  between  the  appar- 
ent brightness  of  the  sun  and  the  stars  is  such  that,  in  former 
times,  men  never  suspected  that  there  could  be  any  similarity 
between  them.  But  the  reader  who  has  carefully  studied  what 
we  have  said  about  these  objects  will  readily  understand  that 
the  sun  is  simply  one  of  the  stars.  In  other  words,  the  stars 
are  suns  like  that  which  gives  us  the  light  of  day.  They  are 
found  to  be  like  the  sun  in  every  feature  that  we  can  discover. 

The  first  question  one  would  ask  on  this  subject  is,  How  does 
the  brightness  of  our  sun  compare  with  that  of  the  stars  ?  To 
answer  this  question  we  must  call  to  mind  on  what  the  bright- 
ness of  a  star  depends.  We  must  distinguish  between  the  real 
brightness  and  the  apparent  brightness  of  such  an  object.  A 
star  of  a  given  real  brightness  looks  fainter  to  us  the  farther 
it  is  away,  just  as  a  distant  gaslight  looks  fainter  than  one 
near  us.  It  was  formerly  supposed  that  the  difference  of 
apparent  brightness  was  due  mostly  to  this  cause,  the  brighter 
stars  being  those  near  us,  and  the  fainter  those  at  a  greater  dis- 
tance. But  it  is  found  that  this  is  not  always  the  case,  and 
that  the  stars  differ  enormously  in  real  brightness.  Sirius,  the 
brightest  star  in  the  heavens,  is  many  times  brighter  than  our 
sun.  Yet  brighter  is  Canopus,  of  which  we  have  already 
spoken.  The  parallax  of  this  star  is  found  to  be  immeasurably 
small,  in  other  words  it  is  immeasurably  far  away.  To  shine 
as  brightly  as  it  does  it  must  be  thousands  of  times  as  bright 
as  the  sun. 

205 


206  ASTRONOMY 

By  comparing  the  light  of  the  sun  and  stars,  and  calling  to 
our  aid  what  knowledge  we  possess  of  the  distance  of  the  latter, 
we  find  that  the  sun  is  quite  a  small  body  compared  with  most 
of  the  brighter  stars  in  the  heavens.  The  latter  are  not  only 
suns,  but  they  are  suns  many  times  brighter  than  ours. 

In  a  few  cases  the  mass  of  a  star  has  been  determined.  Thus 
it  is  found  that  the  mass  of  Sirius  is  several  times  that  of  the 
sun.  But  in  this  case  it  is  known  that  the  light  of  the  star  is 
greater  than  that  of  the  sun  in  a  yet  greater  proportion.  It  is 
thus  found  that  many  of  the  stars  are  much  less  dense  than  the 
sun,  being  probably  like  bubbles,  masses  of  very  hot  gas  in- 
closed in  a  more  or  less  liquid  or  cloudy  envelope. 

This  subject  is  one  of  which  astronomers  are  now  seeking  to 
add  to  their  knowledge.  Such  knowledge  is,  as  yet,  quite  cer- 
tain in  only  a  few  cases.  Even  when  certain  the  result  cannot 
be  stated  with  great  exactness.  We  may  compare  it  to  the 
knowledge  of  a  country  which  one  would  acquire  by  looking 
around  for  a  few  minutes  from  the  top  of  a  high  mountain. 
He  would  see  that  some  villages  were  much  farther  than 
others,  and  would  know  that  here  was  a  river  and  there  a  hill. 
But  if  he  tried  to  make  a  map  of  the  country  he  would  often 
err  widely  in  the  positions  which  he  assigned  to  the  various 
objects  in  the  landscape. 

2.  Proper  Motions  of  the  Stars.  —  We  have  already  said  that 
the  stars  are  generally  in  motion,  often  with  a  very  high  veloc- 
ity. But  their  distance  is  so  great  that  this  motion  would  not 
be  perceptible  to  us  in  a  thousand  years  had  it  not  been  deter- 
mined with  astronomical  instruments  of  great  precision,  espe- 
cially the  Meridian  Circle. 

Such  motions  of  the  stars  are  called  their  proper  motions. 
So  far  as  yet  known  these  motions  take  place  in  straight  lines 
with  a  speed  that  never  varies. 

The  proper  motion  of  a  star  is  detected  by  determining  very 
exactly  its  right  ascension  and  declination  from  time  to  time, 
and  thus  finding  whether  its  position  changes  on  the  celestial 


THE  STARS  AND  NEBULA  207 

sphere.  It  is  found  that  nearly  all  the  brighter  stars,  as  well 
as  many  of  the  faint  ones,  have  a  proper  motion.  We  may 
conclude  from  this  that  every  star  in  the  heavens  is  really 
moving.  In  fact,  if  a  star  were  really  at  rest  at  any  moment, 
the  attraction  of  the  other  stars  would  gradually  start  it  mov- 
ing. In  the  case  of  the  vast  majority  of  the  millions  of  stars 
which  we  see  in  the  heavens,  the  motion  is  so  slow  that  it  has 
not  yet  been  detected. 

We  may  express  the  proper  motion  of  a  star  in  two  ways : 
either  as  so  many  miles  per  second,  which  would  be  its  real 
motion;  or  as  such  an  angle  per  year  or  per  century,  as  seen  by 
us,  which  would  be  its  apparent  motion.  The  real  motions  are 
what  we  should  call  very  rapid,  the  average  being,  probably, 
about  10  miles  a  second.  In  a  year  there  are  31,558,149  sec- 
onds. Hence,  a  star  moving  at  this  average  rate  travels  more 
than  315  millions  of  miles  per  year.  Probably  one  half  the 
stars  are  moving  straight  ahead  with  this  speed,  which,  so  far  as 
we  know,  never  changes,  year  after  year,  or  centu^  after  cen- 
tury. They  are  traveling  forever  on  a  journey  of  which  we 
do  not  know  either  the  beginning  or  the  end.  Yet,  so  vast  is 
their  distance  that  only  the  most  refined  observations  can  show 
how  far  they  move  in  a  whole  year.  The  inconceivable  dis- 
tance we  have  mentioned  is,  to  our  eyes,  a  mere  point  in  the 
sky. 

3.  Motion  of  the  Sun.  —  The  sun,  being  one  of  the  stars,  may 
be  supposed  to  have  a  proper  motion  of  its  own,  as  the  stars 
have.  In  this  case  it  would  carry  the  earth  and  all  the  planets 
with  it,  without  their  relative  positions  being  changed.  It  is 
just  as  if  a  person  were  walking  around  a  chair  in  a  railway 
car  in  motion.  He  could  walk  around  just  as  well  whether 
the  car  were  in  motion  or  had  stopped.  - 

If  a  star  were  at  rest  and  the  sun  in  motion,  the  star  would 
seem  to  us  to  have  a  motion,  on  account  of  the  motion  of  the 
sun.  If  we  were  moving  toward  a  star,  then  the  star  would 
be  shown  by  the  spectroscope  as  if  it  were  moving  toward  us. 


208  ASTRONOMY 

If  the  sun  carrying  the  earth  with  it  were  moving  toward  the 
north  the  star  would  seem  to  be  moving  toward  the  south.  It 
is,  therefore,  impossible  from  any  observation  on  one  star,  to 
determine  how  much  of  the  apparent  motion  of  the  star  is  due 
to  motion  of  the  sun,  and  how  much  to  actual  motion  of  the  star. 
But  if  we  find  that  a  great  majority  of  the  stars  seem  to  be 
moving  in  some  one  direction,  we  should  conclude  that  this 
motion  is  only  apparent,  being  due  to  a  motion  of  the  sun  and 
earth  in  the  opposite  direction. 

This  is  found  to  be  actually  the  case.  A  large  majority  of 
the  stars  are  found  to  be  in  apparent  motion  from  the  direction 
of  the  constellation  Lyra  toward  a  point  in  the  eastern  part  of 
the  constellation  Argo.  This  latter  constellation,  as  we  have 
said,  is  so  far  south  of  the  equator  that  it  only  partly  rises 
above  our  horizon.  We  conclude  from  this  that  our  solar 
system  is  moving  toward  the  constellation  Lyra.  The  velocity 
has  been  determined  in  various  ways,  and  is  found  to  be  about 
10  miles  a  second. 

It  is  in  consequence  of  this  motion  of  our  solar  system  that 
so  many  of  the  stars  seem  to  be  moving  away  from  the  constel- 
lation Lyra.  This  apparent  motion  of  the  stars  is  called  their 
parallactic  motion,  because,  like  parallax,  it  is  due  to  the  change 
of  the  direction  from  which  we  see  them. 

The  motion  of  our  solar  system  toward  the  constellation 
Lyra  is  one  of  the  most  wonderful  conclusions  of  modern 
astronomy.  One  can  get  an  idea  of  the  immensity  of  the 
heavens  by  looking  up  at  the  beautiful  blue  star  Alpha  Lyra 
on  summer  evenings  and  reflecting  that  not  only  during  all 
our  lives,  but  during  the  lives  of  all  our  ancestors  for  untold 
generations  back,  we  have  been  traveling  toward  it  at  the  rate 
of  about  300  millions  of  miles  per  year.  And  yet  the  star 
looks  to  us  as  it  did  to  them.  Kapid  as  the  journey  is,  it  will 
probably  take  our  system  half  a  million  of  years  to  arrive 
where  the  star  now  is.  In  the  meantime  the  latter  will  have 
moved  away,  so  that  it  will  perhaps  be  as  far  away  from  us  as 
it  is  now. 


THE  STARS  AND  NEBULA  209 

4.  Motions  in  the  Line  of  Sight.  —  In  recent  years  the  spec- 
troscope has  made  additions  to  our  knowledge  of  the  motions 
of  the  stars  which  would  have  been  thought  impossible  before 
it  was  invented.    Astronomers  are  now  able,  by  examining  the 
spectrum  of  a  star,  to  determine  how  fast  it  is  approaching  us, 
or  receding  from  us.     The  explanation  of  the  method  belongs 
to  the  subject  of  physics,  but  the  general  principle  is  very 
simple.     If  the  star  is  approaching  us,  the  spectral  lines  will 
all  be  moved  a  little  toward  the  blue  end  of  the  spectrum ;  if 
moving  away  from  us,  they  will  be  displaced  toward  the  red 
end  of  the  spectrum.     So,  what  is  done  with  the  spectroscope 
is  to  compare  the  spectrum  of  a  star  with  that  of  a  substance 
that  gives  the  same  lines  as  the  star.     Thus  it  is  seen  whether 
the  lines  of  a  star  are  displaced  in  one  direction  or  the  other, 
and  how  far,  and  thus  it  is  determined  whether  the  star  is 
moving  toward  or  from  us,  and  how  fast.     The  motion  toward 
or  from  our  system  is  said  to  be  in  the  line  of  sight. 

This  is  now  done  by  photographing  the  spectra  both  of  the 
star  and  of  the  substance  with  which  its  lines  are  compared. 
The  measures  are  then  made  on  the  photographic  negative. 

5.  Distances  of  the  Stars.  —  We  have  already  defined  parallax 
as  difference   of   direction,   and   especially  as   the   difference 
between  the  direction  of  a  heavenly  body  from  the  center  of 
the  earth  and  from  a  point  on  its  surface.     The  distance  of  the 
stars  is  so  great  that  this  difference  of  direction  would  be 


Fio.  109.  —  Annual  parallax  of  a  star. 

entirely  imperceptible  with  any  instrument  that  we  can  make. 
But  as  the  earth  swings  round  the  sun  in  its  vast  orbit,  186 
millions  of  miles  across,  there  must  be  a  corresponding  change 
in  the  direction  of  the  stars  from  us.  This  change  is  called 
annual  parallax,  because  it  goes  through  its  course  in  a  year. 
NEWCOMB'S  ASTRON. — 14 


210  ASTRONOMY 

When  we  speak  of  the  parallax  of  a  star,  we  mean  the  dif- 
ference in  its  direction  as  seen  from  the  sun  and  from  one 
extremity  of  the  earth's  orbit.  This  is  the  angle  which  the 
radius  of  the  earth's  orbit  would  subtend  if  seen  from  the 
star. 

So  small  is  the  annual  parallax,  even  of  the  nearest  star, 
that  astronomers  were  not  able  to  invent  instruments  which 
would  show  it  until  about  the  year  1830.  Then  it  was  found 
by  Bessel  that  a  small  star  of  the  constellation  Cygnus,  called 
61  Cygni,  had  a  parallax  of  about  ^  of  a  second.  About  the 
same  time  it  was  found  that  the  star  Alpha  Centauri,  in  the 
southern  hemisphere,  had  a  still  larger  parallax,  of  which 
the  amount  is  now  known  to  be  about  0.75". 

Since  then  the  parallaxes  of  about  100  stars  have  been 
measured.  But  in  many  cases  it  is  so  small  that  there  is  doubt 
about  the  result.  The  parallaxes  of  two  remarkable  ones  are 
supposed  to  be : 

61  Cygni       .        ...        .        .        0.35" 

Arcturus 0.03" 

Let  us  now  show  how,  from  the  parallax  of  a  star,  we  can 
determine  its  distance.  Let  the  little  circle  EF  be  the  orbit 
of  the  earth  around  the  sun;  S  the  sun,  R  a  star.  From  R 
draw  two  lines,  one  to  the  sun  and  the  other  to  one  extremity 
of  the  orbit.  The  angle  ERS  between  these  lines  will  be  the 
annual  parallax  of  the  star. 


F 

FIG.  110.  — Relation  between  the  parallax  and  distance  of  a  star. 


If  we  draw  a  circle,  as  in  figure  2,  an  arc  of  one  degree  will 

be  about of  the  radius.    That  is,  an  arc  of  57.3°  will  make 

a  length  equal  to  the  radius.     More  exactly,  the  number  of 


THE  STARS  AND  NEBULA  211 

degrees  in  the  radius  is  57.29578.  There  being  60  minutes  in 
a  degree,  and  60  seconds  in  a  minute,  we  shall  find  by  multi- 
plication that  there  are  206265  seconds  in  the  radius. 

Now,  if  we  fancy  ourselves  to  draw  a  circle  with  the  star  as 
a  center  and  a  circumference  passing  through  the  sun,  as  shown 
in  the  dotted  arc  in  figure  110  we  find,  as  already  stated,  that 
the  arc  ES  or  SF,  which  measures  the  parallax,  is  less  than 
one  second.  Therefore,  the  distance  of  a  star,  or  the  radius 
SE,  is  more  than  206265  times  that  of  the  earth  from  the  sun. 
We  may  find  the  exact  distance  by  dividing  206265  by  the 
star's  parallax.  Thus  is  found:  — 

Distance  of  Alpha  Centauri  .  .  .  about  275,000 
Distance  of  61  Cygni  ....  nearly  000,000 
Distance  of  Arcturus  ....  nearly  7,000,000 

These  distances  are  expressed  in  terms  of  the  earth's  dis- 
tance from  the  sun  as  the  measuring  rod.  If  we  wish  to 
express  the  distance  in  miles  we  should  multiply  them  by 
93  millions.  The  distance  of  Arcturus  is  still  uncertain, 
almost  immeasurably  great,  and  the  same  is  true  of  all  but 
about  100  of  the  stars. 

It  is  common  to  express  the  distances  of  the  stars  and  other 
heavenly  bodies  by  the  time  it  takes  their  light  to  travel  to  us. 
The  speed  of  light  is  such  as  it  would  travel  round  the  earth 
more  than  seven  times  in  a  second.  If  we  could  send  a  ray  of 
light  round  by  the  Atlantic  and  Indian  Oceans  to  Australia 
and  back  here  over  the  Pacific,  keep  it  going  round  and  round, 
and  make  a  little  tap  every  time  it  passed  us,  these  taps  would 
follow  each  other  so  rapidly  we  could  hardly  move  our  fingers 
fast  enough  to  make  them. 

Going  at  this  speed  a  ray  of  light  from  the  moon  reaches  us 
in  about  1J  seconds,  and  one  from  the  sun  in  8  min.  20  sec. 
It  reaches  Neptune  from  the  sun  in  4  h.  10  m. 

The  light  from  Alpha  Centauri  reaches  us  in  4£  years; 
that  from  61  Cygni  in  about  9  years.  From  most  of  the 
stars  of  average  brightness  that  we  see  in  the  sky  at  night 


212 


ASTRONOMY 


the  time  is  between  100  and  200  years,  often  more.  From 
the  telescopic  stars  it  ranges  from  a  few  hundred  years  to 
perhaps  several  thousand. 

Real  Speed  of  a  Star.  —  When  we  know  the  distance  of  a  star, 
its  apparent  proper  motion,  and  its  motion  in  the  line  of  sight, 
we  can  determine  the  actual  speed  with  which  it  is  flying 
through  space.  Arcturus  has  the  most  rapid  motion  of  any 
star  visible  to  the  naked  eye.  We  can  easily  compute  it  in 
this  way.  Let  ES  be  the  radius  of  the  earth's  orbit  and  AR 
the  distance  through  which  Arcturus  moves  in  a  year.  The 


FIG.  111. 

angle  between  the  lines  RE  and  RS  is,  as  just  explained,  the 
parallax  of  the  star,  which,  as  we  have  already  said,  is  about 
0.03".  But  the  star  moves  through  an  arc  AR,  which  is  found 
to  be  2.10"  per  year.  This  is  about  70  times  as  much  as  the 
parallax.  It  follows  that  the  star  Arcturus  travels  over  more 
than  70  times  the  distance  of  the  earth  from  the  sun  in  a  year, 
and  therefore  makes  this  distance  in  about  5  days.  If  we 
make  the  calculation  we  shall  find  that  this  corresponds  to  a 
speed  of  more  than  200  miles  per  second ! 

But  this  calculation  takes  no  account  of 
the  motion  of  the  star  in  the  line  of  sight. 
To  show  the  actual  movement,  we  draw  a 
line  AR  at  right  angles  to  the  line  of  sight 
from  the  earth  to  the  star,  and  make  it  pro- 
portional to  the  apparent  motion  as  seen  by 
the  eye.  We  draw  another  line,  RS,  at 
right  angles  to  this,  showing  the  motion  in 
the  line  of  sight.  Then,  joining  AS,  the  hypothenuse  will 
represent  the  total  real  motion  of  the  star. 

In  the  case  of  Arcturus  the  motion  in  the  line  of  sight  is 


FIG.  112. 


THE  STARS  AND  NEBULA  213 

so  small  that  the  hypothenuse  differs  very  little  from  the  line 
AR.  We  may  therefore  regard  200  miles  per  second  as  its 
probable  speed. 

Arcturus  has  undoubtedly  been  flying  through  space  with 
this  wonderful  speed  for  thousands  of  years  in  the  past,  and 
will  continue  its  journey  for  thousands  of  years  in  the  future. 
We  believe  this  to  be  the  case  because  we  cannot  imagine  or 
believe  in  the  existence  of  any  force  which  could  change  so 
rapid  a  motion  of  so  immense  a  body. 

6.  Variable  Stars. — The  great  majority  of  the  fixed  stars 
never  change  in  their  appearance.  But  there  are  some  which 
vary  in  brightness  from  one  time  to  another.  These  are  called 
variable  stars. 

Some  of  these  stars  vary  in  so  irregular  a  way  that  no  law 
can  be  seen  in  their  changes  of  light.  The  most  remarkable 
case  of  this  kind  is  that  of  Eta  Argus,  in  the  southern  celestial 
hemisphere.  Before  1830  it  varied  between  the  second  and 
fourth  magnitudes.  From  1830  to  1843  it  was  sometimes 
nearly  as  bright  as  Sirius.  Then  it  slowly  faded  away  till  it 
became  invisible  to  the  naked  eye.  In  recent  years  it  has 
brightened  up  a  little,  but  in  1898  a  telescope  was  still 
required  to  make  it  visible. 

Most  of  the  variable  stars  go  through  their  changes  accord- 
ing to  a  regular  law,  brightening  up,  and  then  fading  away 
again  in  a  regular  period.  These  are  called  periodic  stars. 

The  phases  of  a  periodic  star  are  the  different  appearances, 
or  degrees  of  brightness,  which  it  exhibits. 

The  period  is  the  length  of  time  in  which  it  goes  through  its 
phases  and  returns  to  the  same  brightness  as  before. 

A  minimum  is  the  phase  when  it  gives  less  light  than  it  did 
just  before,  or  just  afterward. 

A  maximum  is  the  phase  when,  having  been  brightening  up, 
it  is  about  to  fade  again. 

The  law  of  variation  is  not  the  same  in  all  periodic  stars. 
The  various  kinds  of  change  they  go  through  are  called  types. 


214  ASTRONOMY 

Thus,  when  we  say  that  a  star  is  of  the  Algol  type,  we  mean 
that  it  varies  in  the  same  way  that  Algol  does,  a  way  that  will 
be  stated  presently. 

The  period  in  the  case  of  any  one  star  commonly  changes 
slowly  from  time  to  time,  sometimes  being  a  little  longer,  and 
sometimes  a  little  shorter. 

The  periods  of  different  stars  are  very  different  in  length. 
In  the  case  of  some  it  is  only  a  few  hours,  and  in  others  a  few 
days.  In  a  great  number  of  stars  it  ranges  between  six  months 
and  two  years. 

Interesting  Variable  Stars.  —  In  the  great  majority  of  cases 
the  variations  in  the  light  of  these  stars  are  so  slight,  or  the 
stars  are  so  faint,  that  only  the  most  careful  and  well-trained 
observers  would  notice  any  change.  But  there  are  three  of 
which  the  variations  can  be  seen  by  any  one  who  will  take  the 
necessary  care  in  watching. 

The  Star  Beta  Lyrae.  —  One  of  these  stars  is  in  the  constella- 
tion Lyra,  and  is  marked  with  the  Greek  letter  ft  (Beta)  in 
the  figure  of  that  constellation  (p.  203).  One  who  notices  the 
figure  in  the  book  can  easily  recognize  the  star  in  the  heavens. 
If  he  looks  at  it  for  a  few  nights  he  will  find  that  sometimes 
this  star  is  of  the  same  brightness  as  the  star  Gamma,  close 
to  it,  and  sometimes  nearly  a  magnitude  fainter. 

It  goes  through  a  regular  series  of  gradations,  growing 
brighter  and  fainter  at  intervals  of  six  days.  The  curious 
feature  of  this  variation  is  that  at  every  alternate  minimum  it 
is  fainter  than  at  the  adjacent  minima. 

It  is  now  believed  that  this  star  is  composed  of  a  pair  of 
oval-shaped  stars  so  very  close  together  that  they  almost  touch 
each  other.  The  pair  revolve  around  each  other  in  an  orbit  of 
which  the  plane  passes  in  the  direction  of  the  solar  system. 
Hence,  if  we  could  have  a  telescope  powerful  enough  to  see 
what  was  going  on,  we  should,  at  a  certain  time,  see  the  small 
star  pass  in  front  of  the  bright  one,  partially  obscuring  it. 
Six  days  later  the  small  one  would  pass  behind  the  bright  one 
and  be  hidden  by  it,  and  so  on  in  regular  order. 


THE  STARS  AND  NEBULA  215 

The  Star  Algol.  —  Another  remarkable  variable  star  is  Algol, 
or  Beta  Persei.  This  star  is  commonly  between  the  second 
and  third  magnitude  in  brilliancy.  But  it  fades  away  to 
nearly  the  fourth  magnitude  at  intervals  of  about  2  d.  21  h. 
It  takes  3  or  4  hours  to  thus  fade  away,  and  then  in  3  or  4 
hours  more  it  brightens  up  again.  It  is  now  found  that  this  is 
due  to  the  star  having  a  dark  planet  revolving  around  it,  which 
is  nearly  as  large  as  the  star  itself.  Every  time  this  planet 
passes  in  front  of  it  it  obscures  part  of  its  light.  The  fact  of 
this  revolution  has  been  determined  with  the  spectroscope  by 
measuring  the  effect  of  the  eclipsing  planet  upon  the  motion 
of  the  star.  The  planet  itself  is  invisible  even  with  the  most 
powerful  telescope. 

Mira  Ceti.  —  The  third  star  of  this  kind  is  Omicron  Ceti, 
called  also  Mira  Cetij  or  the  wonder  of  Cetus.  It  is  commonly 
invisible  to  the  naked  eye;  but  at  intervals  of  about  11 
months  it  gradually  brightens  up  so  as  to  be  plainly  visible. 
Sometimes  it  attains  the  second  magnitude,  sometimes  only 
the  fifth.  But  however  bright  it  becomes,  it  begins  to  fade 
away  again  in  two  or  three  weeks  and  finally  disappears  from 
view.  It  may,  however,  always  be  seen  with  a  telescope. 

Stars  of  the  Algol  type  are  nearly  always  of  the  same 
brightness,  but  at  certain  regular  intervals  fade  away  for  an 
hour  or  a  few  hours,  and  then  brighten  up  again,  as  we  have 
described  in  the  case  of  Algol.  There  can  be  no  doubt  that 
these  seeming  eclipses  arise  from  the  same  cause  as  those  of 
Algol,  namely,  the  presence  of  a  dark  planet  which  passes 
between  us  and  the  star  at  every  revolution. 

Sometimes  two  stars  of  the  same  kind  revolve  round  each 
other.  In  this  case,  if  the  plane  of  the  orbit  passes  through 
our  solar  system,  we  see  them  mutually  eclipsing  each  other 
at  every  half  revolution.  That  is  to  say,  if  we  call  the  one 
star  A  and  the  other  star  B,  then  at  one  time  of  the  revolution 
A  will  pass  between  us  and  B ;  half  a  revolution  later  A  will 
pass  on  the  other  side  of  B,  so  that  the  latter  will  hide  part  of 
its  light. 


216  ASTRONOMY 

A  remarkable  pair  of  stars  of  this  kind  is  called  Y  Cygni. 
Here  the  pair  looks  like  one  star,  even  in  the  most  powerful 
telescope.  The  way  we  know  there  are  two  stars  is  that  the 
eclipses  occur  at  unequal  intervals.  The  intervals  are  alter- 
nately 44  hours,  28  hours,  44  hours,  and  so  on.  This  regular 
alternation  shows  that  the  two  stars  revolve  round  each  other 
in  an  orbit  having  a  great  eccentricity,  so  that  the  revolution 
is  much  faster  at  one  point  of  the  orbit  than  at  another. 

The  number  of  known  stars  of  this  type  is  very  small. 
Commonly  a  variable  star  does  not  remain  of  the  same  bright- 
ness for  any  considerable  time,  but  varies  in  a  slow  and 
regular  way.  Starting  from  the  time  when  it  is  faintest,  it 
grows  brighter  day  after  day  and  week  after  week,  first  at  a 
very  slow  rate  and  then  more  rapidly,  until  it  attains  its  full 
brightness,  and  then  after  a  few  days  begins  to  fade  away, 
slowly  at  first,  and  afterward  more  quickly,  until  it  gets  back 
to  its  least  brightness,  and  so  on  in  regular  order. 

Several  hundred  variable  stars  are  now  known  to  astrono- 
mers, and  new  discoveries  of  them  are  being  made  very 
rapidly.  There  is  reason  to  believe  that  one  star  out  of  every 
hundred  in  the  heavens  may  vary  to  a  greater  or  less  extent. 

7.  Double  Stars.  —  A  great  many  stars  which  seem  single 
to  the  naked  eye  are  found,  when  viewed  with  a  telescope, 
to  consist  of  two  stars  very  close  together. 
These  are  called  double  stars.  Several  thou- 
sand such  stars  are  known,  and  new  ones  are 
constantly  being  discovered. 

The  first  question  suggested  by  a  pair  of 
this  sort  is  whether  the  two  stars  are  really 
close  together,  or  whether  they  merely  look 
so  because  they  happen  to  lie  in   the  same 
FIG.  118.  —  Orbit  line  from  us.     It  is  now  known  that  nearly 
of  a  double  star.     all  guch    pairs  ape  reajly  dose  together  and 

that  the  two  stars  of  the  pair  revolve  round  each  other.    In 
this  case  the  pair  is  called  a  binary  system. 


THE  STABS  AND  NEBULA  217 

The  time  required  for  one  star  of  a  binary  system  to  make  a 
revolution  round  the  other  is  called  its  period. 

The  period  of  most  binary  systems  is  very  long,  —  several 
centuries,  in  fact.  But  a  few  have  periods  of  less  than  a 
century.  As  accurate  observations  on  these  systems  have  only 
been  made  within  a  hundred  years,  only  these  few  have  been 
seen  to  complete  a  revolution.  Hence  the  exact  time  of  revo- 
lution is  known  only  in  those  cases  in  which  the  period  is  less 
than  a  century,  or  not  much  greater. 

Sometimes  the  two  companion  stars  which  form  a  double 
star,  or  binary  system,  are  nearly  of  the  same  magnitude.  In 
other  cases,  one  star  is  very  much  smaller  than  the  other. 
Indeed,  a  great  many  bright  stars  are  found  to  have  very 
minute  satellites  moving  around  them.  Two  of  the  most 
remarkable  cases  of  this  sort  are  Sirius  and  Procyon,  because 
in  each  case  the  existence  of  the  little  satellite  was  inferred 
by  the  motion  of  the  large  star  produced  by  the  attraction  of 
the  satellite  before  the  latter  was  seen  by  the  telescope. 

In  the  case  of  Sirius,  it  was  found  by  Bessel  and  Peters 
that  the  visible  star  was  moving  round  in  such  a  way  as  to 
show  that  it  was  attracted  by  some  body  very  close  to  it, 
which  they  could  not  see  with  their  telescopes.  But  in  1862 
Mr.  Alvan  Clark  of  Cambridge  made  a  telescope  of  18  inches 
diameter,  which  was  more  powerful  than  any  ever  before  con- 
structed. With  this  he  found  the  companion  of  Sirius  in 
the  same  direction  in  which  it  had  been  predicted. from  the 
motions  of  Sirius  itself.  It  is  now  found  that  a  revolution  is 
made  in  about  50  years. 

The  history  of  Procyon  is  similar.  Observations  for  a  hun- 
dred years  showed  that  it  was  moving  in  a  little  orbit,  as  if 
it  were  attracted  by  a  dark  body  revolving  round  it  in  a  period 
of  40  years.  In  1896  this  little  body  was  actually  found  by 
Professor  Schaeberle  with  the  powerful  telescope  of  the  Lick 
Observatory,  in  California.  It  is  found  that  the  companion 
is  moving  round  Procyon  in  the  same  direction  in  which  the 
motion  had  been  predicted. 


218  ASTRONOMY 

A  few  of  these  binary  stars  will  be  seen  double,  even  in 
quite  a  moderate  telescope.  But  the  greater  number  require  a 
high  telescopic  power  for  their  visible  separation.  The  reason 
of  this  is  that,  in  most  cases,  the  stars  are  so  close  together 
that  they  appear  single  even  with  a  high  magnifying  power, 
while  in  other  cases  the  small  one  is  so  faint  that  it  is  obscured 
by  the  brightness  of  the  larger  one.  With  every  increase  of 
telescopic  power,  it  is  found  that  new  objects  of  this  class  may 
be  seen.  The  greater  number  of  the  difficult  objects  now 
known  would  have  been  entirely  invisible  in  the  largest  tele- 
scopes of  a  hundred  years  ago.  Hence  we  conclude  that, 
if  we  could  increase  our  telescopic  power  without  limit,  we 
should  continually  see  more  and  more  of  these  objects,  and 
find  perhaps  that  every  star  in  the  heavens  had  other  stars 
revolving  around  it. 

An  interesting  question  now  arises.  Since  our  sun  has  a 
retinue  of  eight  dark  planets  revolving  around  it,  may  it  not 
be  that  all  the  stars  have  planets  which  we  cannot  see  on 
account  of  their  immense  distance?  This  is  quite  possible. 
If  we  could  fly  away  from  the  solar  system,  and  carry  with  us 
the  most  powerful  telescope  ever  made,  all  the  planets,  even 
the  brightest,  would  become  invisible  through  our  telescope, 
one  by  one,  long  before  we  got  halfway  to  the  nearest  star. 
Even  Jupiter  does  not  give  -nnrhnw  part  as  much  light  as 
the  sun,  and  we  can  readily  understand  that  a  star  giving 
only  I00oooo  as  much  light  as  another  would  be  totally  invisi- 
ble to  us.  The  fact  that  we  cannot  see  planets  revolving 
around  the  stars,  therefore,  proves  nothing. 

It  might  seem  to  us  that  it  would  be  forever  impossible 
that  men  living  on  this  globe  should  be  able  to  detect  among 
the  stars  bodies  which  are  forever  invisible.  And  yet  this 
wonderful  thing  is  being  done  through  the  researches  with 
the  spectroscope  and  observations  on  the  variable  stars.  We 
have  already  mentioned  the  variable  stars  of  the  Algol  type, 
which  are  partially  eclipsed  by  the  revolution  of  dark  bodies 
around  them.  Such  stars  will  appear  variable  to  us  only  in 


THE  STARS  AND  NEBULA  219 

the  rare  cases  when  the  plane  of  the  orbit  of  the  dark  body 
passes  near  the  direction  of  the  solar  system.  If  the  plane 
should  be  in  a  different  position,  then  the  dark  body,  or  the 
revolving  star,  would  not  seem  to  us  to  pass  over  the  bright 
one  at  each  revolution,  but  would  pass  above  or  below  it,  as 
the  moon  at  conjunction  with  the  sun  may  pass  above  or  below 
it  without  eclipsing  it.  In  these  cases  the  bright  star  will  not 
be  a  variable  one,  and  our  telescopes  would  give  no  indication 
that  there  was  more  than  one  body. 

But  the  spectroscope  now  comes  in  and  tells  the  story  of 
companions  which  are  and  must  forever  be  entirely  invisible 
to  human  eyes.  Systems  made  known  in  this  way  are  called 
spectroscopic  binary  systems,  or  merely  spectroscopic  binaries. 

There  are  two  ways  in  which  a  star  may  be  shown  to  form 
part  of  a  binary  system  by  means  of  the  spectroscope. 

One  way  consists  in  measuring  the  motion  of  the  star  in  the 
line  of  sight.  Sometimes  this  motion  is  found  to  vary  in  a 
regular  period,  increasing  for  a  certain  time,  then  decreasing, 
then  increasing  again,  and  so  on.  Sometimes  it  will  move 
toward  us  for  a  certain  interval,  then  nearly  stop,  then  come 
toward  us  again.  There  can  be  but  one  possible  cause  for  such 
a  change  of  motion,  the  attraction  of  a  body  revolving  around 
the  star.  In  such  a  case  each  body  would  revolve  around  the 
common  center  of  gravity  of  both,  as  we  have  described  in  the 
case  of  the  sun  and  moon. 

The  other  method  rests  on  the  same  principle.  We  have 
said  that  the  motion  in  the  line  of  sight  is  shown  by  a  slight 
displacement  of  the  lines  of  the  spectrum.  Sometimes  these 
lines  are  seen  to  be  double  for  a  period ;  then  single ;  then 
double  again  in  regular  order.  This  arises  from  the  fact  that 
there  are  two  stars  moving  around  each  other  and  showing  the 
same  lines.  When  one  is  moving  toward  us  and  the  other  from 
us  a  spectral  line  of  one  star  is  displaced  in  one  direction  and 
the  corresponding  line  of  the  other  is  displaced  in  the  opposite 
direction.  Then  two  lines  are  seen  instead  of  one.  But  when 
the  stars  move  toward  or  from  us  with  the  same  velocity  only 


220  ASTRONOMY 

one  line  is  seen,  because  the  lines  of  the  two  stars  are  merged 
together. 

The  periods  of  these  spectroscopic  binary  stars  are  generally 
only  a  few  days,  whereas,  as  we  have  already  mentioned,  those 
which  we  see  double  in  the  telescope  have  periods  of  many 
years.  But  we  may  suppose  that  binary  systems  having  all 
intermediate  periods  really  exist,  though  they  cannot  be  seen 
double  in  the  telescope.  It  is  only  a  few  years  since  these  sys- 
tems began  to  be  discovered  with  the  spectroscope,  and  we  have 
not  had  time  to  detect  those  having  a  long  period.  Besides, 
such  a  system  will  not  be  seen  even  with  a  spectroscope,  unless 
the  invisible  body  which  produces  the  motion  has  a  great  mass. 
If  an  astronomer  on  a  distant  star  should  observe  our  sun  with 
a  spectroscope  he  would  never  be  able  to  detect  any  motion  due 
to  the  attraction  of  the  planets  which  revolve  around  it. 

We  may  therefore  conclude  that  planets  are  probably  revolv- 
ing around  great  numbers  of  stars ;  but  up  to  the  present  time 
astronomers  have  not  been  able  to  learn  much  more  about  them 
than  what  we  have  just  set  forth.  But  it  is  wonderful  that  we 
should  ever  know  anything  at  all  about  them. 

8.  Clusters  and  Nebulae.    Clusters  of  Stars.  — We  have  already 
spoken  of  some  of  these  clusters  which  can  be  seen  by  the 
80UTH  naked  eye,  either  as  separate 

stars  or  as  patches  of  milky 
light.  Quite  a  number  of 
them  are  seen  in  the  Milky 
Way,  especially  the  southern 
part,  which  is  visible  in  the 
late  summer  or  autumn. 

Some  of  these  clusters  are 
among  the  most  interesting 
telescopic  objects.  One  of 
the  most  remarkable  is  the 

FIG.   114. -Star  cluster  47  Toucani,     Sreat   cluster    of    Hercules, 
as  drawn  by  Sir  John  Herschel.         This  is  formed  of  thousands 


THE  STARS  AND  NEBULA 


221 


FIG.  115.  —  Star  cluster  w  Centauri, 
as  drawn  by  Sir  John  Herschel. 


of  stars  in  such  close  proximity  that  they  can  scarcely  be  dis- 
tinguished even  in  the  most  powerful  telescopes.  When  we 
look  at  this  object,  we  might 
imagine  it  to  be  a  little  colony 
on  the  outskirts  of  creation 
itself,  the  inhabitants  of  which 
might  hold  communication  with 
each  other,  even  if  they  lived 
on  different  planets.  Yet, 
if  any  of  the  stars  which 
form  this  cluster  have  planets 
revolving  round  them,  it  is 
quite  likely  that  their  distance 
apart  is  so  great  that  the  in- 
habitants of  one  planet  would 
know  no  more  about  the  other 
planets,  or  the  other  stars,  then  we  know  about  Venus  or  Mars. 

Nebulas.  —  There  are  in  the  heavens  a  great  number  of  objects 
which  appear  in  the  telescope  as  very  faint  masses  of  soft  dif- 
fused light,  like  thin  pieces  of  cloud.  Such  a  mass  is  called  a 
nebula  (Latin  for  cloud). 

A  few  of  these  objects  are  visible  to  the  naked  eye,  but  the 
greater  number,  comprising  many  thousands,  can  be  seen  only 
with  a  good  telescope  and  on  very  clear  nights.  Sometimes 
when  a  cluster  of  stars  is  viewed  with  the  naked  eye,  or  with  a 
telescope  which  will  not  enable  us  to  distinguish  the  separate 
stars,  it  has  the  appearance  of  a  nebula.  Hence  it  was  once 
questioned  whether  all  these  objects  might  not  really  be  clus- 
ters of  stars.  This  question  has  been  settled  in  recent  times 
by  the  spectroscope,  which  shows  that  the  greater  number  of 
the  nebulae  are  masses  of  glowing  gas,  and  therefore  cannot  be 
made  up  of  stars. 

The  most  wonderful  object  of  this  sort  is  the  great  nebula 
of  Orion.  It  is  plainly  visible  to  the  naked  eye,  to  which  it 
has  the  appearance  of  a  star  of  the  fourth  magnitude  with  a 
somewhat  indistinct  and  hazy  outline.  It  is  the  middle  star  of 


222  ASTRONOMY 

the  three  which  form  the  sword  of  Orion,  hanging  below  the 
belt,  and  may  be  found  on  the  figure  of  that  constellation 
already  given.  When  examined  in  a  good  telescope  a  curious 
dark  rift  or  cavity  is  seen  near  one  edge.  In  this  rift  are  a 
number  of  stars,  of  which  four  are  much  brighter  than  the 
others.  These  are  called  the  trapezium.  These  stars  give  us 
the  impression  of  being  formed  out  of  the  matter  of  the  nebula 


FIG.  116.  —  The  annular  nebula  of  Lyra. 

which  perhaps  once  filled  the  rift.  There  are  a  great  number 
of  other  stars  in  and  around  the  nebula,  some  of  which  are 
believed  to  be  variable.  Two  very  small  stars  are  inside  the 
trapezium. 

Another  of  these  objects  easily  visible  to  the  naked  eye  is 
the  great  nebula  of  Andromeda.  This  does  not  look  like  a 
star,  but  can  easily  be  seen  as  a  small  hazy  patch  of  light  of 
an  elliptical  form.  It  is  sometimes  mistaken  for  a  small 


THE  STARS  AND  NEBULA 


223 


comet.     With  a  small  telescope  it  looks  like  a  patch  of  fog 
illuminated  by  a  lantern  behind  it  or  in  it. 

Many  nebulae  are  of  singular  or  fantastic  shapes.  A  cele- 
brated one  is  the  annular  or  ring  nebula  of  Lyra,  situated  in 
that  constellation  about  halfway  between  the  stars  Beta  and 
Gamma.  The  circumference  is  so  much  brighter  than  the  cen- 


FIG.  117.  —  The  Omega  Nebula,  as  drawn  by  Sir  John  Herschel. 


ter  that,  to  the  telescopes  of  former  times,  it  seemed  to  be  a  true 
elliptical  ring.  But  with  our  large  telescopes  the  whole  inte- 
rior appears  to  be  filled  with  nebulous  light,  as  shown  in  the 
figure.  The  figures  show  several  other  curiously  formed  neb- 
ulae, as  drawn  by  Sir  William  Herschel  and  others.  It  is  now 
found  that  there  are  many  thousand  nebulae  which  are  too  faint 
to  be  seen  even  with  a  telescope,  but  which  can  be  photographed 


224  ASTRONOMY 

by  several  hours*  exposure.     One  of  these  is  near  the  Pleiades. 
Another  very  large  one  is  in  the  constellation  Orion. 

It  is  thought  that  stars  and  systems  are  formed  by  the  grad- 
ual cooling  and  condensation  of  nebulae.  In  this  way  the  re- 
markable regularity  of  the  solar  system  is  explained.  It  is 
supposed  that  the  matter  composing  the  sun  and  planets  was 


FIG.  118. —The  Trifid  Nebula. 

once  a  mass  of  glowing  gas,  with  a  slow  revolution  on  its  axis. 
As  this  gas  cooled,  the  outer  portion  condensed  into  a  ring. 
As  the  central  portion  grew  smaller,  additional  rings  were 
formed.  Finally,  each  ring  contracted  into  a  planet,  and  the 
central  mass  into  the  sun. 

This  view  of  the  origin  of  the  solar  system  is  called  the 
nebular  hypothesis. 


CHAPTER   XVI 
A  BRIEF  HISTORY  OF  ASTRONOMY 

IT  is  interesting  to  know  what  men  thought  of  the  earth  and 
the  heavens  before  their  true  relation  and  the  laws  of  the  celes- 
tial motions  were  understood. 

In  very  ancient  times  it  was  well  known  to  philosophers  and 
navigators  that  the  earth  was  a  globe.  Navigators  could  see 
how,  on  the  Mediterranean  Sea,  the  sail  of  a  distant  ship  would 
gradually  sink  below  the  horizon,  owing  to  the  roundness  of 
the  surface  of  the  sea.  Observers  of  the  heavens  knew  that 
when  they  traveled  south  the  stars  behind  them  would  sink 
below  the  horizon,  while  new  stars  in  front  of  them  would 
rise.  They  also  knew  that  an  eclipse  of  the  moon  which  was 
seen  after  sunset  at  Babylon  occurred  at  an  earlier  hour  at 
points  farther  west.  All  this  proved  to  them  that  the  earth 
on  which  we  live  is  a  globe. 

What  they  did  not  know,  and  had  no  means  of  finding  out, 
was  that  this  globe  turned  on  its  axis  and  moved  round  the 
sun.  It  is  sometimes  supposed  that  Pythagoras  and  other 
ancient  philosophers  taught  this  system;  but  this  cannot  be 
proved,  because  none  of  their  writings  have  come  down  to  us. 
The  earliest  astronomer  who  has  told  us  much  of  the  ancient 
ideas  of  astronomy  was  Ptolemy,  who  nourished  at  Alexandria 
about  the  year  150  of  our  era.  He  wrote  a  great  work  called 
the  Syntaxis,  but  more  commonly  known  as  Almagest,  from  an 
Arabic  expression  signifying  The  Great  Work.  The  system  of 
astronomy  which  we  find  in  this  book  is  commonly  called  the 
Ptolemaic  System,  after  this  writer.  Its  main  doctrines  are 
these  :  — 

NEWCOMB'S  ASTRON. — 15        226 


226  ASTRONOMY 

1.  The  earth  is  a  globe. 

2.  This  globe  is  at  rest  in  the  center  of  the  heaven. 

3.  The  heaven  is  spherical  in  form. 

4.  The  earth  is  so  much  smaller  than  the  heaven  that  it  is 

only  a  point  in  comparison. 

5.  The  heaven  makes  a  revolution  around  the  earth  every 

day. 

It  is  interesting  to  notice  what  is  right  and  what  is  wrong 
in  these  five  propositions.  The  first  and  fourth,  as  we  already 
know,  are  quite  correct ;  and  the  fact  that  the  ancients  were 
able  to  learn  so  much  about  the  respective  magnitudes  of  the 
earth  and  the  heaven  is  very  remarkable.  The  second  is 
wrong.  Properly  speaking,  there  is  no  such  thing  as  the 
center  of  the  heaven.  But  the  ancients,  seeing  the  apparent 
circular  motion  of  the  stars,  and  noticing  how  they  seemed 
to  be  spread  out  on  the  celestial  sphere,  supposed  the  heaven 
itself  to  be  spherical. 

The  third  and  fifth  propositions  are,  of  course,  all  wrong. 

To  us  it  is  much  more  reasonable  to  suppose  that  a  com- 
paratively small  point  like  the  earth  is  in  motion  and  the 
much  larger  heaven  at  rest,  than  it  is  to  suppose  the  reverse. 
The  ancients  thought  it  was  the  heaven  and  not  the  earth 
which  moved,  because  they  did  not  suppose  the  former  to 
consist  of  the  same  kind  of  matter  as  the  earth,  and  because 
they  were  not  acquainted  with  the  laws  of  motion.  They 
supposed  that  there  was  an  inherent  tendency  in  all  moving 
bodies  to  stop  moving  and  come  to  rest.  They  reached  this 
conclusion  because  that  seemed  to  be  the  case  with  all  bodies 
in  motion  around  us.  What  they  did  not  see  was  that  this 
tendency  to  stop  was  not  inherent  in  the  motion  itself,  but 
arose  from  the  fact  that  the  motions  of  all  bodies  around  us 
were  constantly  resisted  by  friction,  the  resistance  of  the  air, 
and  the  contact  with  the  earth. 

Some  ancient  philosophers  did  really  suggest  that  it  was  the 
earth  which  turned  on  its  axis  while  the  heaven  remained  at 
rest.  But  Ptolemy  thought,  that  if  the  earth  revolved  on  its 


A  BRIEF  HISTOEY  OF  ASTRONOMY  22T 

axis,  it  must  be  turning  with  such  speed  that  it  would  leave 
the  air  behind  it,  so  that  we  should  have  a  furious  gale  blowing 
from  east  to  west.  He  could  not  conceive  of  earth,  atmosphere, 
and  everything  on  the  earth  running  so  smoothly  together  that 
we  should  be  quite  unconscious  of  any  motion  at  all. 

In  watching  the  stars  the  ancient  philosophers  saw  what  to 
them  was  a  curious  fact,  namely,  that  the  stars  preserved  their 
relative  positions  to  each  other  while  they  seemed  to  turn 
round  the  earth,  just  as  if  they  were  set  in  a  hollow  revolving 
sphere.  So  they  imagined  such  a  sphere,- of  which  the  sub- 
stance was  the  purest  and  most  translucent  crystal,  and  which 
was  therefore  called  the  crystalline  sphere.  This  sphere  they 
fancied  to  turn  on  an  axis  passing  through  the  center  of  the 
earth  and  coinciding  with  what  we  know  to  be  the  earth's 
axis.  This  was  called  the  axis  of  the  heaven.  The  sphere, 
in  turning,  was  supposed  to  carry  the  stars  with  it 

But  this  sphere  did  not  account  for  the  motions  of  the 
planets.  By  the  planets  the  ancients  meant  all  the  heavenly 
bodies,  seven  in  number,  which  did  not  seem  to  revolve  in  the 
supposed  crystalline  sphere.  These  bodies,  as  we  have  already 
said,  were  the  Sun,  the  Moon,  Mercury,  Venus,  Mars,  Jupiter, 
and  Saturn. 

Each  of  these  planets  was  supposed  to  have  its  own  sphere. 
They  quite  correctly  supposed  that  the  spheres  of  the  planets 
must  be  inside  the  sphere  of  the  fixed  stars ;  but  they  had  no 
idea  how  much  farther  the  stars  were  than  the  planets. 

They  also  noticed  that  the  planets,  excepting  the  sun  and 
moon,  moved  in  the  celestial  sphere,  sometimes  from  west 
toward  east  and  sometimes  from  east  toward  west.  They 
accounted  for  this  by  supposing  them  to  have  two  motions, 
one  round  the  earth,  and  the  other  on  an  epicycle.  The  latter 
was  a  small  circle  the  center  of  which  was  considered  to  move 
round  the  earth,  while  the  planet  moved  around  in  the  circle 
itself. 

We  now  know  that  this  apparent  motion  round  the  epicycle 
is  really  due  to  the  motion  of  the  earth  round  the  sun,  which 


228 


ASTRONOMY 


makes  the  apparent  motion  of  the  planet  sometimes  retrograde 
and  sometimes  direct.  But  the  ancients  supposed  it  to  be  a 
real  motion.  This  notion  was  held  until  the  sixteenth  cen- 
tury, when  it  was  refuted  by  the  great  Copernicus. 

Copernicus  was  born  at  Thorn,  in  Poland,  in  1473.  He 
studied  at  the  University  of  Cracow  and  became  a  priest.  He 

also  acted  for  a  short  time 
as  Professor  of  Mathe- 
matics in  Rome.  He 
thought  for  many  years 
over  the  mystery  of  the 
heavenly  motions,  and  saw 
clearly  how  easily  they 
could  be  explained  by 
supposing  that  the  earth 
revolved  round  the  sun 
and  turned  on  its  axis, 
instead  of  its  being  the 
sun  and  the  heavens  that 
moved.  He  spent  many 
years  in  writing  a  great 
work  in  which  this  view 
was  set  forth,  and  all  the 
calculations  growing  out 
of  it  were  made.  But  he  was  so  modest  and  so  fearful  of  the 
prejudice  that  might  be  excited  by  a  new  system  that  he 
resisted  all  the  entreaties  of  friends  to  publish  his  book,  until 
he  was  about  seventy  years  old.  Then  it  was  printed  un<lrr 
the  title  De  Revolutionib'us  Orbium  Ccelestium.  Copernicus  died 
in  the  year  1543,  on  the  very  day  that  he  received  the  first 
printed  copy  of  his  book. 

The  system  here  set  forth,  which  we  now  know  to  be  the 
true  one,  is  commonly  called  the  Copemican  System.  Sometimes 
it  is  called  the  Heliocentric  System  because  it  makes  the  sun 
the  center  of  motion;  while  the  old  Ptolemaic  one  is  called 
the  Geocentric  System  because  the  earth  is  the  center  of  motion. 


FIG.  119. — Copernicus. 


A   BRIEF  HISTORY  OF  ASTRONOMY 


229 


For  a  hundred  years  after  the  death  of  Copernicus  his  sys- 
tem was  not  generally  accepted.  The  Church  authorities 
feared  that  it  was  contrary  to  the  Scriptures,  and  so  were  dis- 
posed to  forbid  its  being  taught  as  a  truth,  although  they  were 
willing  astronomers  should  use  it  in  their  calculations,  merely 
assuming  it  to  be  true.  About  1620  Galileo  was  tried  and 
imprisoned  for  publishing  books  in  which  the  truth  of  the 
system  was  set  forth.  But 
this  did  not  prevent  in- 
telligent men  from  believ- 
ing in  it. 

About  1580  arose  a 
celebrated  astronomical 
observer,  Tycho  Brahe. 
He  built  a  great  observa- 
tory at  a  place  which  he 
called  Uraniberg,  on  an 
island  near  Copenhagen. 
Through  the  patronage  of 
the  king  of  Denmark  he 
was  enabled  to  fit  up  his 
establishment  with  instru- 
ments of  a  size  and  pre- 
cision before  unknown. 
Unfortunately  he  did  not 
fully  accept  the  Coperni- 
can  system  of  astronomy, 

and  in  consequence  his  fame  is  not  so  great  as  it  otherwise 
would  be.  What  is  still  more  unfortunate  is  that  the  tele- 
scope had  not  then  been  invented,  and  consequently  his  obser- 
vations were  not  exact  enough  to  be  of  use  to  his  successors. 
Yet  by  their  aid  Kepler,  who  was  a  contemporary  of  Galileo, 
showed  that  the  orbit  of  Mars  round  the  sun  was  an  ellipse, 
having  the  sun  in  one  of  its  foci.  Hence  the  laws  of  motion 
of  the  planets  in  ellipses,  which  we  have  already  mentioned, 
are  called  Kepler's  laws. 


FIG.  120. —Kepler. 


230 


ASTRONOMY 


About  1680,  as  we  have  already  said,  Sir  Isaac  Newton  showed 
that  the  motions  of  the  heavenly  bodies  could  be  all  explained 
by  the  theory  of  universal  gravitation.  English  philosophers 
accepted  this  view  immediately,  but  those  on  the  continent  of 
Europe  were  slow  to  follow.  Descartes,  another  philosopher, 
claimed  that  the  planets  were  carried  around  the  sun,  and  the 

satellites  round  the  plan- 
ets, by  their  floating  in  an 
etheral  medium  which  was 
kept  in  rotation  like  a 
whirlpool.  Hence  this 
view  is  called  the  theory 
of  vortices.  It  was  exten- 
sively held,  but  was  at 
last  abandoned  when  it 
became  clear  that  the  cor- 
rect theory  was  that  of 
gravitation.  It  now  be- 
came a  very  interesting 
problem  with  mathemati- 
cians to  demonstrate  math- 
ematically how  the  planets 
ought  to  move  under  the 
influence  of  the  sun's  at- 
traction combined  with 
their  attraction  on  each  other.  One  of  the  greatest  men  in  this 
work  was  Laplace,  who  lived  at  Paris  during  the  latter  part  of 
the  eighteenth  and  the  beginning  of  the  nineteenth  century. 
He  published  a  great  work  called  the  Mtcanique  C&este,  in 
which  the  methods  of  solving  the  problem  were  set  forth. 

Let  us  again  mention  the  four  greatest  books  in  which  the 
laws  of  the  celestial  motions  have  been  expounded  :  — 
Ptolemy's  Syntaxis,  commonly  called  Almagest; 
Copernicus  De  Revolutionibus  Orbium  Coelestium; 
Newton's  Principia; 
Laplace's  Mdcanique  Celeste. 


FIG.  121.  —  Sir  Isaac  Newton. 


A   BRIEF  HISTORY  OF  ASTRONOMY 


231 


FIG.  122.  —  Galileo. 


Observational  Astronomy. 
—  We  may  now  say  some- 
thing of  the  history  of  tele- 
scopes and  observational 
astronomy.  Before  the  time 
of  Galileo,  astronomers  had 
to  make  all  their  observa- 
tion? with  the  naked  eye, 
and  with  very  crude  and 
very  inaccurate  instru- 
ments. The  first  telescopes 
were  made  in  Holland 
about  the  year  1608;  but 
they  were  only  very  imper- 
fect little  spyglasses,  and  it 
does  not  seem  that  their 

makers  ever  thought  of  looking  at  the  heavens  with  them. 
The  telescope  is  commonly  thought  to  have  been  reinvented 

by  Galileo,  who  heard 
that  such  an  instrument 
had  been  made  in  Hol- 
land, and  began  to  study 
how  it  must  have  been 
constructed.  Whether  he 
really  invented  it  over 
again  without  help,  in  this 
way,  or  whether  he  saw  a 
description  of  it  is  not 
certain.  What  is  certain 
is  that  he  was  the  first 
person  to  explain  its  prin- 
ciples, and  to  point  it  at 
the  heavens  and  show 
what  wonders  it  would 
make  known  to  men. 
FIG.  123. -Laplace.  With  his  little  install- 


232 


ASTRONOMY 


ments,  poor  as  they  were,  he  saw  that  the  Milky  Way  was 
made  up  of  countless  stars.  He  also  saw  the  phases  of 
Venus,  which  proved  that  the  planet  was  a  dark  globe  which 
we  see  by  the  light  of  the  sun.  He  saw  the  rings  of  Saturn 
projecting  from  the  planet  like  two  little  handles,  but  could 
not  see  that  they  were  rings.  He  saw  the  satellites  of  Jupiter, 
and  found  that  they  revolved  round  Jupiter  as  the  planets 

did  round  the  sun. 

Huyghens,  who  nour- 
ished during  the  latter 
half  of  the  seventeenth 
century,  was  the  first  to 
show  the  true  form  of  the 
rings  of  Saturn. 

The  telescope  has  been 
gradually  perfected  and 
enlarged  from  the  time  of 
Galileo  until  the  present. 
The  greatest  step  forward 
was  made  by  the  cele- 
brated Sir  William  Her- 
schel,  who  observed  in 
England  during  the  latter 
part  of  the  eighteenth  cen- 
tury. He  made  reflect- 
ing telescopes  many  times 
larger  than  any  before 
made,  and  nearly  as  large  as  any  made  up  to  our  time.  With 
them  he  studied  the  starry  heavens,  and  made  greater  discov- 
eries than  any  one  had  done  before  him. 

The  first  maker  of  an  achromatic  telescope  was  Dollond  of 
London,  who  worked  about  1760.  But  his  glasses  were  only  a 
few  inches  in  diameter,  because  the  art  of  making  flint  glass  of 
good  quality  had  not  then  been  mastered.  During  the  early 
part  of  the  nineteenth  century  the  most  celebrated  maker  of 
refracting  telescopes  was  Fraunhofer,  of  Germany.  During  its 


FIG.  124.  —  Sir  William  Herschel. 


ASTRONOMICAL    WORK  AT  THE  PRESENT  TIME     233 

later  part,  from  1860  to  1890,  the  place  of  Fraunhofer  was 
taken  by  Alvan  Clark,  of  Cambridgeport,  Massachusetts,  and 
his  two  sons,  Alvan  and  George.  We  have  already  spoken  of 
the  object  glasses  made  by  this  family.  Now,  there  are  many 
able  constructors  of  large  telescopes  in  Europe  and  America. 


ASTRONOMICAL   WORK   AT  THE   PRESENT  TIME 

The  application  of  the  spectroscope  to  astronomical  observa- 
tion, which  commenced  about  the  year  1860,  has  given  rise  to 
a  new  branch  of  astronomy  known  as  astrophysics.  This 
branch  has  been  powerfully  reenforced  by  the  application  of 
photography  to  astronomical  observation.  A  telescope  may 
be  used  like  a  very  large  camera.  If  we  point  it  at  the  heav- 
ens and  then  place  a  sensitized  plate  in  its  focus,  as  the  pho- 
tographer puts  such  a  plate  in  the  focus  of  his  camera,  we 
may  take  a  picture  of  any  heavenly  body  at  which  the  tele- 
scope is  pointed.  If  we  point  the  photographic  telescope  at  the 
sky  during  the  night,  and  make  it  follow  the  stars  in  their 
diurnal  motion,  we  may  thus  take  a  picture  of  the  stars  in  the 
field  of  view.  The  number  of  stars  on  the  plate  will  depend 
on  the  length  of  the  exposure.  It  is  found  that  when  the 
telescope  is  so  set  as  to  follow  the  diurnal  motion  of  the  stars 
very  exactly,  the  image  of  the  same  stars  may  be  kept  on  the 
same  point  of  the  plate  during  a  period  of  several  hours.  In 
this  case  photographs  will  be  found  of  many  more  stars  than 
the  eye  can  see  with  the  same  telescope.  In  other  words,  the 
power  of  the  telescope  is  greatly  increased  by  the  extreme 
sensitiveness  of  the  photographic  plate. 

We  have  already  spoken  of  some  remarkable  nebulae  found 
in  this  way  which  would  never  have  been  known  had  we 
depended  on  the  eye  alone.  Photography  is  now  applied  with 
success  to  spectroscopy  by  throwing  the  spectrum  of  a  star 
upon  a  sensitized  plate.  A  photograph  can  thus  be  made  of  a 
spectrum  which  the  eye  could  scarcely  see.  This  photograph 
can  be  studied  by  the  astronomer  at  his  leisure,  and  many  con- 


234  ASTRONOMY 

elusions  deduced  from  it  which  he  would  not  be  able  to  deduce 
by  a  study  of  the  spectrum  itself  with  his  eye. 

It  is  possible  to  discover  new  objects  in  the  heavens  by  pho- 
tography with  much  greater  facility  than  by  the  eye.  We 
have  already  seen  how  minor  planets  are  discovered  by  photog- 
raphy. No  astronomer  now  attempts  to  find  them  in  any  other 
way.  He  simply  points  his  telescope  at  any  region  of  the 
heavens  near  the  ecliptic,  starts  its  clockwork  so  that  it  shall 
exactly  follow  the  stars,  and  takes  his  photograph  by  an  expo- 
sure of  several  hours.  Then,  if  there  is  any  planet  in  the  field 
of  view,  it  will  not  be  photographed  as  a  star,  which  is  a  mere 
point,  but  as  a  short  line.  This  is  because  the  planet  has 
moved  on  the  celestial  sphere  and  left  its  trace  upon  the  plate. 

During  an  eclipse  of  the  sun  a  few  years  ago,  the  impression 
of  a  little  comet  was  found  on  the  photographic  plate,  in  the 
immediate  neighborhood  of  the  sun.  What  became  of  it  is  not 
known,  because  it  was  never  seen  by  the  eye.  In  one  case  a 
comet  was  actually  discovered  by  photography  and  afterward 
observed  by  the  eye. 

In  1887  an  arrangement  was  made  among  a  number  of 
observatories  in  the  northern  and  southern  hemispheres  to 
make  a  complete  photographic  map  of  the  heavens.  To  each 
observatory  a  certain  region  of  the  heavens  was  assigned. 
When  this  work  is  completed  there  will  exist  a  set  of  several 
thousand  photographs  showing  the  starry  heavens  as  they 
were  about  the  end  of  the  nineteenth  century.  How  many 
millions  of  stars  may  be  impressed  on  these  plates  we  cannot 
yet  say.  Each  region  is  photographed  twice  in  order  that 
there  may  be  no  doubt  of  the  existence  of  anything  that  looks 
like  a  star  on  the  photographic  plate.  Were  only  one  plate 
taken,  it  is  possible  that  sometimes  a  speck  might  be  mistaken 
for  a  star.  But  if  the  same  speck  appears  on  two  plates,  then 
we  know  that  a  star  must  have  been  there.  If  then,  at  any 
future  time,  another  photograph  of  the  region  is  taken,  it  can 
be  determined  whether  any  star  has  disappeared  or  whether 
a  new  one  has  come  into  sight 


ASTRONOMICAL    WORK  AT  THE  PRESENT  TIME     235 

Quite  separate  from  this  international  work  is  that  of  the 
Harvard  Observatory  at  Cambridge,  Massachusetts.  Here  a 
constant  watch  is  kept  on  the  heavens  every  clear  night  by  a 
moving  photographic  telescope,  which  records  automatically 
any  new  object  as  bright  as  the  sixth  magnitude  that  may 
come  into  view.  Photographs  of  large  regions  of  the  heavens 
are  also  taken  very  often,  so  that  changes  in  the  brightness  of 
the  stars  or  planets  before  unknown  may  be  detected. 

Ever  since  the  beginning  of  their  science  astronomers  have 
been  studying  the  motions  of  the  heavenly  bodies  with  a  view 
of  establishing  the  laws  that  govern  them  and  the  causes  that 
may  change  them,  and  tracing  the  conclusions  that  may  thus  be 
drawn  respecting  the  structure  of  the  universe.  We  have  told 
the  main  results  in  the  preceding  chapters  of  this  book,  but 
there  are  many  important  questions  which  we  have  not  had 
time  to  discuss  or  explain.  One  is,  whether  the  motions  of  the 
planets  are  influenced  by  any  force  except  the  gravitation  of 
the  sun  and  of  the  other  planets.  Mathematical  methods  have 
been  brought  to  such  perfection  that  the  astronomer,  when 
aided  by  exact  observations,  can  predict  the  path  which  a 
planet  ought  to  follow  in  consequence  of  the  attraction  of  all 
known  bodies,  with  great  exactness,  for  many  years  ahead. 
Then,  comparing  his  conclusions  as  to  the  place  of  the  planet 
with  observations,  he  can  see  whether  his  predictions  are  cor- 
rect. If  they  are,  he  knows  that  the  theory  on  which  they  are 
based  is  true.  If  the  predicted  positions  do  not  agree  with 
observation,  he  investigates  the  cause.  We  have  seen  what  a 
brilliant  result  was  thus  obtained  in  the  discovery  of  the  planet 
Neptune. 

There  are  some  cases  in  which  success  has  not  yet  been 
reached.  We  recall  that  the  orbits  of  all  the  planets  slowly 
change  their  positions  in  consequence  of  the  attraction  of  each 
planet  on  all  the  others.  It  is  found  that  the  perihelion  of 
Mercury  changes  its  position  somewhat  more  than  it  should 
in  consequence  of  the  attraction  of  the  other  planets.  For  some 
time  it  was  supposed  that  this  might  be  due  to  the  attraction 


236  ASTRONOMY 

of  unknown  planets  between  Mercury  and  the  sun.  But  it  is 
now  made  almost  certain  that  no  planets  of  sufficient  mass  to 
produce  the  effect  can  exist.  The  cause  of  the  motion  is  there- 
fore still  unknown. 

It  is  also  found  that  the  motion  of  the  moon  changes  very 
slowly  and  slightly  from  one  century  to  another,  sometimes 
going  a  little  ahead,  sometimes  falling  a  little  behind.  The 
cause  of  these  deviations  has  not  yet  been  discovered. 

Altogether,  although  so  much  has  been  learned  about  the 
heavenly  bodies,  there  is  still  so  much  left  to  leai'n  that  the 
most  advanced  astronomer  must  feel  as  if  he  were  only  at 
the  threshold  of  his  science. 


INDEX 


Including  references  to  definitions  of  the  technical  terms  used  in  this  book 


Aberration,  60,  78,  79. 

Absorption,  selective,  74. 

Achromatic,  61. 

Aldebaran,  the  star,  199. 

Almagest  of  Ptolemy,  225. 

Altitude,  16. 

Andromeda,  the  constellation,  195. 

Angle  of  the  vertical,  91. 

Annual  motion,  34. 

Annual  parallax,  209. 

Annular  eclipse,  129. 

Annulus,  129. 

Antarctic  circle,  36. 

Antipodes,  11. 

Aphelion,  142. 

Apogee,  120. 

Apparent  diameter,  74. 

Apparent  motion,  20. 

Apparent  noon,  51. 

Apparent  time,  51. 

Apparition,  circle  of  perpetual,  26. 

Arctic  circle,  36. 

Aspects  of  a  planet,  146. 

Asteroids,  144. 

Astronomical  latitude,  91. 

Astronomical  refraction,  58. 

Astronomical  time,  139. 

Astrophysics,  233. 

Atmosphere,  100. 

Atmospheric  refraction,  58. 

Auriga,  the  constellation,  195, 198. 

Autumnal  equinox,  37,  41. 

Axis,  11. 

Axis,  declination,  63;  polar,  63. 

Base  line,  94. 

Bayer,  system  of  naming  stars,  196. 

Betelguese,  the  star,  200, 


Binary  systems,  216,  219. 
Body,  80. 

Calendar,  133 ;  Gregorian,  135;  Julian, 
135. 

Canals  on  Mars,  157. 

Cancer,  tropic  of,  36. 

Canis  Major,  the  constellation,  200. 

Canis  Minor,  the  constellation,  200. 

Capella,  the  star,  198. 

Capricorn,  tropic  of,  37. 

Cassiopeia,  the  constellation,  195,  197 

Castor  and  Pollux,  200. 

Celestial  equator,  21. 

Celestial  horizon,  17. 

Celestial  poles,  21. 

Celestial  sphere,  12. 

Central  eclipse,  128. 

Centrifugal  force,  89. 

Chromatic  aberration ,  60. 

Circle,  antarctic,  36;  arctic,  36; 
meridian,  70. 

Circle  of  perpetual  apparition,  26. 

Circle  of  perpetual  occultation,  28. 

Circumpolar,  197. 

Civil  time,  50,  139. 

Clark,  Alvan,  66,  233. 

Clock,  sidereal,  49. 

Clusters  of  stars,  195. 

Collimator,  72. 

Coma  of  a  comet,  176. 

Comet,  Biela's,  185 ;  description  of  a, 
176 ;  Donati's,  of  1858, 182 ;  Encke's, 
183;  Halley's  180;  great,  of  1843, 
182 ;  great,  of  1882, 183 ;  periodic,  177. 

Comets,  constitution  of,  185 ;  orbits  of, 
178 ;  remarkable,  180 ;  telescopic,  177. 

Conjunction,  117,  147. 

237 


238 


INDEX 


Constant  of  aberration,  79. 
Constellation,  195. 
Coperuican  System,  228. 
Copernicus,  work  of,  228. 
Corona  of  sun,  130. 
Counter-glow,  102. 
Crystalline  sphere,  227. 
Cycle,  Metonic,  137;  solar,  138. 

Day,  11, 133;  sidereal,  49. 

Declination,  28. 

Declination  axis,  63 ;  parallel  of,  30. 

Diameter,  apparent,  74. 

Direct  motion,  148. 

Dispersion,  58. 

Distance,    apparent,    13;   linear,  13; 

mean,  141. 
Diurnal  motion,  20. 
Dollond,  232. 
Dominical  letter,  137. 
Double  stars,  216. 
Draco,  constellation,  197. 

Earth,  magnitude  of  the,  90. 

Easter  Sunday,  134. 

Eastern  time,  54. 

Eccentricity,  141. 

Eclipse,  partial,  125. 

Eclipses  of  the  moon,  124. 

Eclipses  of  the  sun,  126. 

Ecliptic,  39 ;  obliquity  of  the,  35 ;  plane 

of,  35 ;  pole  of  the,  46. 
Ellipticity,  90. 
Elongation  of  a  planet,  146. 
Equation  of  time,  51. 
Equator,  11 ;  celestial,  21 ;  of  the  sun, 

108;  plane  of  the,  20. 
Equatorial  telescope,  62. 
Equinox,  autumnal,  37,  41 ;  vernal,  3(5. 
Equinoxes,  precession  of  the,  45. 
Equinoxial  year,  44. 
Eros,  the  planet,  101. 
Eyepiece,  61. 

Field  of  view,  62. 

Focal  length,  60. 

Focus  of  an  elipse,  141. 

Force,  80;  centrifugal,  89. 

Fraunhofer,  66,  232. 

Friction,  80. 

Full  moon,  119;  Paschal,  134. 


Galileo  invents  telescope,  231. 
Gegenschein,  102. 
Gemini,  constellation,  200. 
Geocentric  latitude,  92. 
Geocentric  System,  228. 
Geodesy,  93. 
Georgium  sidus,  172. 
Geographical  latitude,  91. 
Geoid,  90. 
Gibbous,  118. 
Golden  number,  137. 
Gravitation,  universal,  83. 
Gravity,  10,  81. 
Gregorian  Calendar,  135. 

Head  of  a  comet,  176. 
Heliocentric  System,  228. 
Hemisphere,  invisible,  19;  visible,  19. 
History  of  Astronomy,  225. 
Horizon,    celestial,   15;    dip   of,  16; 

plane  of ,  15. 

Horizontal  parallax,  76. 
Hour  circle,  28,  30. 
Hyades,  199. 
Hypothesis,  nebular,  224. 

Image,  60. 

Inertia,  82. 

Inferior  conjunction,  147. 

Inferior  planets,  144. 

Invisible  hemisphere,  19. 

Julian  Calendar,  135. 
Jupiter,  the  planet,  162 ;  satellites  of, 
165 ;  surface  of,  163. 

Kepler,  work  of,  229. 
Kepler's  laws,  141. 

Latitude,  astronomical,  91 ;  geocentric, 
92 ;  geographical,  91 ;  parallel  of,  30 

Letter,  Dominical,  137. 

Libration,  120. 

Light,  zodiacal,  101. 

Line,  of  nodes,  126, 153;  of  sight,  62; 
of  total  eclipse,  127. 

Line,  vertical,  17. 

Linear  distance,  13. 

Local  time,  53. 

Longitude,  telegraphic,  97. 

Lunar  month,  134. 


INDEX 


239 


Major  planets,  142. 

Mars,  canals  of,  157;  the  planet,  156; 

rotation  of,  159;  satellites  of,  159; 

supposed  inhabitants  of,  159. 
Mass,  84. 
Matter,  80. 

Maximum  of  a  variable  star,  213. 
Mean  distance,  141. 
Mean  noon,  51. 
Mean  sun,  51. 
Mean  time,  51. 
Mercury,  the  planet,  151 ;  transits  of, 

153. 

Meridian,  22. 
Meridian  circle,  70. 
Meteoric  showers,  188. 
Meteoroids,  187. 
Meteors,  187. 
Metonic  cycle,  137. 
Mecanique  Celeste,  Laplace's,  230. 
Mile,  nautical,  93;  statute,  93. 
Minimum  of  a  variable  star,  213. 
Minor  planets,  144. 
Moon,  eclipse  of  the,  124. 
Motion,    annual,    34;    apparent,    20; 

direct,  148;  diurnal,  20;  laws  of,  81 ; 

proper,  192;  relative,  10;  retrograde, 

148. 

Mountain  time,  54. 
Mounting,  02. 
Month,  lunar,  134. 

Nadir,  16. 

Nautical  mile,  93. 

Neap  tides,  123. 

Nebula,  221. 

Nebular  hypothesis,  224. 

Neptune,  the  planet,  173 ;  satellite  of, 

174. 

New  moon,  118. 
New  style,  136. 
Newton,  Sir  Isaac,  83,  230. 
Nodes,  126 ;  line  of,  153. 
Noon,  apparent,  51 ;  mean,  51. 
Noon,  sidereal,  49. 
Nucleus,  106, 176. 

Object  glass,  61. 

Objective,  61. 

Obliquity  of  the  ecliptic,  35. 

Occultation,  circle  of  perpetual,  28. 


Olbers's  hypothesis,  160. 
Old  style,  136. 
Opposition,  147. 
Orbits  of  the  planets,  163. 
Orion,  the  constellation,  200. 

Parallax,  75,  76,  209. 

Parallel,  of  declination,  30,  of  lati- 
tude, 30. 

Partial  eclipse,  125. 

Pacific  time,  54. 

Paschal  full  moon,  134. 

Pegasus,  square  of,  204. 

Penumbra,  in  eclipse,  123 ;  of  sua  spot, 
106. 

Perigee,  120. 

Perihelion,  142. 

Period,  of  a  binary  star,  217,  of  a  va- 
riable star,  213. 

Periodic  comet,  177. 

Periodic  stars,  213. 

Periodic  time,  141,  144. 

Perseids,  190. 

Perturbations,  150.  • 

Phases,  of  an  eclipse,  129;  of  a  vari- 
able star,  213. 

Photography,  celestial,  233. 

Photosphere,  104. 

Plane,  of  the  ecliptic,  35;  of  the  equa 
tor,  20. 

Planet,  primary,  144. 

Planets,  33 ;  191 ;  inferior,  144 ;  major, 
142;  minor,  144;  relative  size  of, 
143 ;  secondary,  144 ;  superior,  144. 

Pleiades,  199. 

Polar  axis,  63. 

Poles,  11 ;  celestial,  21  ;  of  the  ecliptic, 
46;  of  the  sun,  107. 

Polestar,  24. 

Precession  of  the  equinoxes,  45. 

Primary  planet,  144. 

Principia  of  Newton,  230. 

Procyon,  the  star,  20Q. 

Prominences  of  the  sun,  109. 

Proper  motion,  192. 

Ptolemaic  System,  225. 

Radiant  point,  188. 
Radius  vector,  141. 
Reflecting  telescope,  64. 
Refraction,  5(5,  58. 


240 


INDEX 


Relative  motion,  10. 

Retrograde  motion,  148. 

Revolution,  sidereal,  117 ;  synodic,  118. 

Rigel,  the  star,  200. 

Right  ascension,  29. 

Saturn,  the  planet,  167 ;  views  of,  167 ; 

satellites  of,  170. 
Secondary  planets,  144. 
Secular  variations,  150. 
Selective  absorption,  74. 
Semidiameter,  75. 
Shadow,  123. 
Shadow  cone,  123. 
Shooting  stars,  187. 
Showers,  meteoric,  188. 
Sidereal  clock,  49. 
Sidereal  day,  49. 
Sidereal  noon,  49. 
Sidereal  revolution,  117. 
Sidereal  time,  49. 
Sidereal  year,  45. 
Signs  of  the  zodiac,  40. 
Sirius,  the  star,  200. 
Solar  cycle,  138. 
Solar  spectrum,  71. 
Solar  system,  33. 
Solar  year,  44. 

Solstice,  summer,  41 ;  winter,  41. 
Spectroscope,  71. 

Spectroscopic  binary  systems,  219. 
Spectrum,  71 ;  of  a  star,  71 ;  solar,  71. 
Spectrum  analysis,  71. 
Sphere,  celestial,  12. 
Spring  tides,  122. 
Standard  time,  53. 
Stars,  shooting,  187;  telescopic,  192. 
Stationary,  148. 
Statute  mile,  93. 
Style,  new,  136;  old,  136. 
Sun  distance,  145. 
Sun,  eclipses  of  the,  126;  mean,  51; 

corona   of,    129;    density    of,    102; 

equator  of,  108;  heat  of,  105;  mass 

of,  105;  rotation  of,  107;  spots  on, 

106. 
Superior  conjunction,  147. 


Superior  planets,  144. 
Synodic  revolution,  118. 
Syntaxis  of  Ptolemy,  225. 

Tail  of  a  comet,  176. 

Taurus,  the  constellation,  198. 

Telegraphic  longitude,  97. 

Telescope,  equatorial,  62. 

Telescopic  comets,  177. 

Telescopic  stars,  192. 

Tidal  waves,  121. 

Tides,  121-123. 

Time,  apparent,  51 ;  astronomical,  139, 
central,  54;  civil,  50,  139;  eastern, 
54;  equation  of ,  51 ;  local,  53;  mean, 
51;  Pacific,  54;  periodic,  141,  144; 
mountain,  54  ;  sidereal,  49. 

Total  eclipse,  line  of,  127. 

Transit  instrument,  68. 

Tropic,  of  Cancer,  36;  of  Capricorn, 
37. 

Tycho  Brahe,  observations  of,  229. 

Umbra,  of  sun  spot,  106. 

Uranus,  the  planet,  172 ;  satellites  of, 

172. 

Ursa  major,  the  constellation,  195, 197. 
Ursa  minor,  the  constellation,  197. 

Variable  stars,  213. 

Variations,  secular,  150. 

Venus,  the  planet,  153;  rotation  of, 

155 ;  transits  of,  155. 
Vernal  equinox,  36. 
Vertical,  angle  of  the,  91 
Vertical  line,  17. 
Vector,  radius,  141. 

Waves,  tidal,  121. 
Weight,  84. 

Year,  equinoxial,  44;  sidereal,  45; 
solar,  44. 

Zenith,  16;  distance,  17. 
Zodiac,  39 ;  signs  of,  40. 
Zodiacal  light,  101. 


YB  35975 


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